Don't overwrite existing headers when mapping agentInfo headers into elasticsearch output (#9199)

* Don't overwrite existing headers.

* Add changelog.

Author: blakerouse

changelog/fragments/1753906482-Don't-overwrite-elasticsearch-output-headers-from-enrollment---headers-flag.yaml

@@ -0,0 +1,32 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: bug-fix
+
+# Change summary; a 80ish characters long description of the change.
+summary: Don't overwrite elasticsearch output headers from enrollment --headers flag
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+#description:
+
+# Affected component; usually one of "elastic-agent", "fleet-server", "filebeat", "metricbeat", "auditbeat", "all", etc.
+component: elastic-agent
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+pr: https://github.com/elastic/elastic-agent/pull/9199
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+issue: https://github.com/elastic/elastic-agent/issues/9197

pkg/component/component.go

@@ -591,7 +591,10 @@ func toIntermediate(policy map[string]interface{}, aliasMapping map[string]strin
 				}
 
 				for headerName, headerVal := range agentHeaders {
-					headers[headerName] = headerVal
+					// only set headers for those that are not already set
+					if _, ok := headers[headerName]; !ok {
+						headers[headerName] = headerVal
+					}
 				}
 
 				output[headersKey] = headers

pkg/component/component_test.go

@@ -2166,7 +2166,8 @@ func TestToComponents(t *testing.T) {
 						"type":    "elasticsearch",
 						"enabled": true,
 						"headers": map[string]interface{}{
-							"header-two": "val-2",
+							"header-two":   "val-2",
+							"header-three": "val-3",
 						},
 					},
 				},
@@ -2195,8 +2196,9 @@ func TestToComponents(t *testing.T) {
 							Config: MustExpectedConfig(map[string]interface{}{
 								"type": "elasticsearch",
 								"headers": map[string]interface{}{
-									"header-two": "val-2",
-									"header-one": "val-1",
+									"header-two":   "val-2",
+									"header-one":   "val-1",
+									"header-three": "val-3",
 								},
 							}),
 						},
@@ -2214,7 +2216,8 @@ func TestToComponents(t *testing.T) {
 				},
 			},
 			headers: &testHeadersProvider{headers: map[string]string{
-				"header-one": "val-1",
+				"header-one":   "val-1",
+				"header-three": "val-3-diff",
 			}},
 		},
 		{
@@ -2227,6 +2230,7 @@ func TestToComponents(t *testing.T) {
 						"enabled": true,
 						"headers": map[string]interface{}{
 							"cloud1": "beat1",
+							"cloud3": "beat3",
 						},
 					},
 				},
@@ -2257,6 +2261,7 @@ func TestToComponents(t *testing.T) {
 								"headers": map[string]interface{}{
 									"cloud1": "beat1",
 									"cloud2": "beat2",
+									"cloud3": "beat3",
 								},
 							}),
 						},
@@ -2275,6 +2280,7 @@ func TestToComponents(t *testing.T) {
 			},
 			headers: &testHeadersProvider{headers: map[string]string{
 				"cloud2": "beat2",
+				"cloud3": "beat3-diff",
 			}},
 		},
 		{