Add ability to configure dynamic auth headers

[zeitlinger]

Spec issue: open-telemetry/opentelemetry-specification#1344

The new declarative config doesn't support the use of dynamic auth headers - which is possible now

What would be needed?

A way to update the config model in a customizer with a transient part - i.e. that can't be read from yaml, in this case a function that returns the headers when invoked.

Possible solutions

Placeholder

Use a placehoder syntax to reference system properties (or something similar)

This would have a similar syntax as env vars (see https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/data-model.md#environment-variable-substitution)

For example, ${secret:API_KEY} would access a secret storage and we'd have to add setter methods for secrets.

Would roughly work like that

    sectrectStorage.set("GCP", () -> getRequiredHeaderMap(credentials, configProperties));
    add.forEach(
        (key, value) -> {
          if (headers.stream().noneMatch(header -> key.equals(header.getName()))) {
            headers.add(new NameStringValuePairModel().withName("${secret:GCP").withValue(value));
          }
        });

Authentication provider

Make authentication provider a top-level construct - like it already is for otel collector: https://opentelemetry.io/docs/collector/building/authenticator-extension/

The config would be

exporter:
  otlp_http: 
    auth:
      authenticator: 
        gcp:
          region: foo #provider specific settings here

is similar to collector today, e.g.

receivers:
  otlp/auth:
    protocols:
      grpc:
         endpoint: 0.0.0.0:4317
         auth:
           authenticator: oidc

Prior Art / Related

Collector

Collector uses a server interface that is similar (only has the go typical context added): https://github.com/open-telemetry/opentelemetry-collector/blob/main/extension/extensionauth/server.go

[zeitlinger]

Should be added to Declarative Configuration: Java - but I don't have permissions

[zeitlinger]

Should be added to Declarative Configuration: Java - but I don't have permissions

adding from the project works apparently