Allow configuring OTEL_* settings from anywhere, not only environment

[srogovtsev]

#141 (see also #12) has brought support for standard OpenTelemetry configuration (OTEL_EXPORTER_OTLP_TRACES_ENDPOINT and so on), but it only consumes them from environment, while standard OpenTelemetry implementation in .NET allow consuming the same configuration options from passed IConfiguration, no matter where they are defined (which is handy for non-standard deployments). I propose we add some way to pass any configuration to the OpenTelemetry configuration method.

As I understand that adding IConfiguration would not be desirable due to an additional dependency, I propose we simply expose already existing Func<string, string?> configuration getter (which is very easily mappable to IConfiguration.Item[string], to allow for additional extensibility.

[mcdevmc]

Is there a way to make this solution work when reading Serilog configuration from appsettings.json?

When the WriteTo is configured in code, I can read the OTEL_* variables successfully from appsettings.json with the following:

builder.Services.AddSerilog((services, logging) =>
{
    logging.ReadFrom.Configuration(builder.Configuration)
        .ReadFrom.Services(services)
        .WriteTo.OpenTelemetry(options => { }, getConfigurationVariable: key => builder.Configuration[key]);
});

but not sure how/if the WriteTo can be configured in appsettings.json:

{
  "Serilog": {
    "Using": [
      "Serilog.Sinks.Console",
      "Serilog.Sinks.OpenTelemetry"
    ],
    "MinimumLevel": {
      "Default": "Information",
      "Override": {
        "Microsoft.AspNetCore": "Warning"
      }
    },
    "WriteTo": [
      { "Name": "Console" },
      {
        "Name": "OpenTelemetry",
        "Args": {
          "getConfigurationVariable": ?
        }
      }
    ],
    "Enrich": [ "FromLogContext" ]
  },
  "OTEL_EXPORTER_OTLP_ENDPOINT": "http://localhost:5341/ingest/otlp",
  "OTEL_EXPORTER_OTLP_PROTOCOL": "http/protobuf",
}

For now, the only solution I've been able to find is to explicitly duplicate the values into the Endpoint and Protocol Args for the sink. This is less ideal since I still need to keep the OTEL_* variables for trace logs, so I need to define the values twice.

[nblumhardt]

@mcdevmc thanks for the note; this might be a good one to discuss in a new thread. I don't think there's an easy answer to this right now; about all I can come up with is to add some special handling for getConfigurationVariable in Serilog.Settings.Configuration, which would be tough to get across the line. More options might come up if we dig into it.