[feature] add support for sending headers to tracing system

[joschi]

Description

Add support for sending additional HTTP or gRPC headers which can be used for authentication or other additional information for the tracing system without having to set up a local instance of the OpenTelemetry Collector to add these headers.

Example with Dash0:

tracing-enabled: false
tracing-transport: "grpc"
tracing-endpoint: "ingress.eu-west-1.aws.dash0.com:4317"
tracing-headers:
  "Authorization": "Bearer DASH0_AUTH_TOKEN"
  "Dash0-Dataset": "gotosocial"

Example with Honeycomb:

tracing-enabled: false
tracing-transport: "grpc"
tracing-endpoint: "api.honeycomb.io:443"
tracing-headers:
  "x-honeycomb-team": "YOUR_API_KEY"
  "x-honeycomb-dataset": "YOUR_DATASET"

refs #1230

See also https://pkg.go.dev/go.opentelemetry.io/otel/exporters/otlp/otlptrace/[email protected]#WithHeaders.

Checklist

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• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
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[joschi]

note: help would be appreciated here for a proper translation.

[tsmethurst]

Oh this looks interesting! :)

[daenney]

I actually wonder if we shouldn't just tell people to use the OTLP environment variables instead. Otherwise we're going to have to add configuration options over time for anything the SDK already supports.

For example, you could set those values already with OTEL_EXPORTER_OTLP_TRACES_HEADERS and they should get picked up. By explicitly using WithHeaders we actually disable that facility.

I'm not entirely sure where to draw the line with this. Realistically, we can remove the tracing-* config options entirely since everything we have can already be done using the OTLP_* environment variables.