feat: No longer launch Go-based agent for compatibility/OTLP/AAP config (#788)

https://datadoghq.atlassian.net/browse/SVLS-7398

- As part of coming release, bottlecap agent no longer launches Go-based
agent when compatibility/AAP/OTLP features are active
- Emit the same metric when detecting any of above configuration
- Update corresponding unit tests
- Refactor the references of enhanced metrics.

Manifests:
- [Test lambda
function](https://us-east-1.console.aws.amazon.com/lambda/home?region=us-east-1#/functions/ltn1-fullinstrument-bn-cold-python310-lambda?code=&subtab=envVars&tab=testing)
with
[logs](https://us-east-1.console.aws.amazon.com/cloudwatch/home?region=us-east-1#logsV2:log-groups/log-group/$252Faws$252Flambda$252Fltn1-fullinstrument-bn-cold-python310-lambda/log-events/2025$252F08$252F21$252F$255B$2524LATEST$255Df3788d359677452dad162488ff15456f$3FfilterPattern$3Dotel)
showing compatibility/AAP/OTPL are enabled
<img width="2260" height="454" alt="image"
src="https://github.com/user-attachments/assets/5dfd4954-5191-4390-83f5-a8eb3bffb9d3"
/>

-
[Logging](https://app.datadoghq.com/logs/livetail?query=functionname%3Altn1-fullinstrument-bn-cold-python310-lambda%20Metric&agg_m=count&agg_m_source=base&agg_t=count&cols=host%2Cservice&fromUser=true&messageDisplay=inline&refresh_mode=paused&storage=driveline&stream_sort=desc&viz=stream&from_ts=1755787655569&to_ts=1755787689060&live=false)
<img width="1058" height="911" alt="image"
src="https://github.com/user-attachments/assets/629f75d1-e115-4478-afac-ad16d9369fa7"
/>

-
[Metric](https://app.datadoghq.com/screen/integration/aws_lambda_enhanced_metrics?fromUser=false&fullscreen_end_ts=1755788220000&fullscreen_paused=true&fullscreen_refresh_mode=paused&fullscreen_section=overview&fullscreen_start_ts=1755787200000&fullscreen_widget=2&graph-explorer__tile_def=N4IgbglgXiBcIBcD2AHANhAzgkAaEAxgK7ZIC2A%2BhgHYDWmcA2gLr4BOApgI5EfYOxGoTphRJqmDhQBmSNmQCGOeJgIK0CtnhA8ObCHyagAJkoUVMSImwIc4IMhwT6CDfNQWP7utgE8AjNo%2BvvaYRGSwpggKxkgA5gB0kmxgemh8mAkcAB4IHBIQ4gnSChBoSKlswAAkCgDumBQKBARW1Ai41ZxxhdSd0kTUBAi9AL4ABABGvuPAA0Mj4h6OowkKja2DCAAUAJTaCnFx3UpyoeEgo6wgsvJEGgJCN3Jk9wrevH6BV-iWbMqgTbtOAAJgADPg5MY9BRpkZEL4UHZ4LdXhptBBqNDsnAISAoXp7NDVJdmKMfiBsL50nBgOSgA&refresh_mode=sliding&from_ts=1755783890661&to_ts=1755787490661&live=true)
<img width="1227" height="1196" alt="image"
src="https://github.com/user-attachments/assets/2922eb54-9853-4512-a902-dfa97916b643"
/>

Author: litianningdatadog

bottlecap/src/bin/bottlecap/main.rs

@@ -515,7 +515,7 @@ async fn extension_loop_active(
         enhanced_metrics::new(metrics_aggr_handle.clone(), Arc::clone(config));
 
     // Send config issue metrics
-    let config_issues = config::fallback(config);
+    let config_issues = config::inspect_config(config);
     send_config_issue_metric(&config_issues, &lambda_enhanced_metrics);
 
     let propagator = Arc::new(DatadogCompositePropagator::new(Arc::clone(config)));

bottlecap/src/config/mod.rs

@@ -441,12 +441,12 @@ impl Default for Config {
     }
 }
 
-fn log_fallback_reason(reason: &str) {
+fn log_unsupported_reason(reason: &str) {
     error!("Fallback support for {reason} is no longer available.");
 }
 
 #[must_use = "fallback reasons should be processed to emit appropriate metrics"]
-pub fn fallback(config: &Config) -> Vec<String> {
+pub fn inspect_config(config: &Config) -> Vec<String> {
     let mut fallback_reasons = Vec::new();
 
     // Customer explicitly opted out of the Next Gen extension
@@ -458,15 +458,15 @@ pub fn fallback(config: &Config) -> Vec<String> {
 
     if opted_out {
         let reason = "extension_version";
-        log_fallback_reason(reason);
+        log_unsupported_reason(reason);
         fallback_reasons.push(reason.to_string());
     }
 
     // ASM / .NET
     // todo(duncanista): Remove once the .NET runtime is fixed
     if config.serverless_appsec_enabled && has_dotnet_binary() {
         let reason = "serverless_appsec_enabled_dotnet";
-        log_fallback_reason(reason);
+        log_unsupported_reason(reason);
         fallback_reasons.push(reason.to_string());
     }
     // OTLP
@@ -501,7 +501,7 @@ pub fn fallback(config: &Config) -> Vec<String> {
 
     if has_otlp_config {
         let reason = "otel";
-        log_fallback_reason(reason);
+        log_unsupported_reason(reason);
         fallback_reasons.push(reason.to_string());
     }
 
@@ -724,15 +724,98 @@ pub mod tests {
     };
 
     #[test]
-    fn test_reject_on_opted_out() {
+    fn test_baseline_case() {
         figment::Jail::expect_with(|jail| {
             jail.clear_env();
-            jail.set_env("DD_EXTENSION_VERSION", "compatibility");
             let _config = get_config(Path::new(""));
             Ok(())
         });
     }
 
+    #[test]
+    fn test_inspect_config() {
+        struct TestCase {
+            name: &'static str,
+            env_vars: Vec<(&'static str, &'static str)>,
+            yaml_content: Option<&'static str>,
+            expected_reasons: Vec<&'static str>,
+        }
+
+        let test_cases = vec![
+            TestCase {
+                name: "default config - no fallback reasons",
+                env_vars: vec![],
+                yaml_content: None,
+                expected_reasons: vec![],
+            },
+            TestCase {
+                name: "extension_version compatibility - should fallback",
+                env_vars: vec![("DD_EXTENSION_VERSION", "compatibility")],
+                yaml_content: None,
+                expected_reasons: vec!["extension_version"],
+            },
+            TestCase {
+                name: "otlp config enabled - should fallback",
+                env_vars: vec![(
+                    "DD_OTLP_CONFIG_RECEIVER_PROTOCOLS_GRPC_ENDPOINT",
+                    "localhost:4317",
+                )],
+                yaml_content: None,
+                expected_reasons: vec!["otel"],
+            },
+            TestCase {
+                name: "multiple fallback reasons",
+                env_vars: vec![
+                    ("DD_EXTENSION_VERSION", "compatibility"),
+                    ("DD_OTLP_CONFIG_METRICS_ENABLED", "true"),
+                ],
+                yaml_content: None,
+                expected_reasons: vec!["extension_version", "otel"],
+            },
+            TestCase {
+                name: "otlp config via yaml - should fallback",
+                env_vars: vec![],
+                yaml_content: Some(
+                    r"
+                    otlp_config:
+                      receiver:
+                        protocols:
+                          grpc:
+                            endpoint: localhost:4317
+                ",
+                ),
+                expected_reasons: vec!["otel"],
+            },
+        ];
+
+        for test_case in test_cases {
+            figment::Jail::expect_with(|jail| {
+                jail.clear_env();
+
+                // Set environment variables
+                for (key, value) in &test_case.env_vars {
+                    jail.set_env(key, value);
+                }
+
+                // Create YAML file if provided
+                if let Some(yaml_content) = test_case.yaml_content {
+                    jail.create_file("datadog.yaml", yaml_content)?;
+                }
+
+                let config = get_config(Path::new(""));
+                let fallback_reasons = inspect_config(&config);
+
+                assert_eq!(
+                    fallback_reasons, test_case.expected_reasons,
+                    "Test case '{}' failed: expected {:?}, got {:?}",
+                    test_case.name, test_case.expected_reasons, fallback_reasons
+                );
+
+                Ok(())
+            });
+        }
+    }
+
     #[test]
     fn test_fallback_on_otel() {
         figment::Jail::expect_with(|jail| {
@@ -849,17 +932,6 @@ pub mod tests {
         });
     }
 
-    #[test]
-    fn test_allowed_but_disabled() {
-        figment::Jail::expect_with(|jail| {
-            jail.clear_env();
-            jail.set_env("DD_SERVERLESS_APPSEC_ENABLED", "true");
-
-            let _config = get_config(Path::new(""));
-            Ok(())
-        });
-    }
-
     #[test]
     fn test_precedence() {
         figment::Jail::expect_with(|jail| {
@@ -871,7 +943,7 @@ pub mod tests {
             ",
             )?;
             jail.set_env("DD_SITE", "datad0g.com");
-            let config = get_config(Path::new(""));
+            let config = get_config(Path::new("")).expect("should parse config");
             assert_eq!(config.site, "datad0g.com");
             Ok(())
         });