Support automatic baggage propagation from gRPC metadata

[o-shevchenko]

Description

Spring Boot's management.tracing.baggage.remote-fields configuration automatically propagates baggage from HTTP headers to OpenTelemetry spans. However, this functionality does not work for gRPC metadata headers, requiring manual implementation via a ServerInterceptor.

Expected Behavior

When configuring baggage propagation in application.yml:

management:
  tracing:
    baggage:
      enabled: true
      remote-fields: x-request-id, x-user-id, x-tenant-id
      tag-fields: x-request-id, x-user-id, x-tenant-id

The specified fields from gRPC metadata should be automatically extracted and added as span tags/baggage, similar to how it works for HTTP headers in Spring Boot applications.

Current Behavior

The baggage configuration only works for HTTP requests. When sending gRPC requests with metadata headers:

grpcurl -plaintext -v \
  -H 'x-request-id: trace-check-success-002' \
  -H 'x-tenant-id: test-tenant-dev-001' \
  -d '{"name": "test"}' \
  localhost:9090 \
  MyService/MyMethod

The headers (x-request-id, x-tenant-id) do not appear in OpenTelemetry span tags.

Current Workaround

A manual ServerInterceptor is required to extract metadata and add it to spans:

@GlobalServerInterceptor
class GrpcHeaderInterceptor(
    private val tracer: Tracer,
    private val tracingProperties: TracingProperties
) : ServerInterceptor {

    override fun <ReqT, RespT> interceptCall(
        call: ServerCall<ReqT, RespT>,
        headers: Metadata,
        next: ServerCallHandler<ReqT, RespT>
    ): ServerCall.Listener<ReqT> {
        val currentSpan = tracer.currentSpan()
        val remoteFields = tracingProperties.baggage.remoteFields
        val tagFields = tracingProperties.baggage.tagFields

        remoteFields.forEach { headerName ->
            val key = Metadata.Key.of(headerName, Metadata.ASCII_STRING_MARSHALLER)
            headers.get(key)?.let { value ->
                tracer.createBaggageInScope(headerName, value)

                if (tagFields.contains(headerName)) {
                    currentSpan?.tag(headerName, value)
                }
            }
        }

        return next.startCall(call, headers)
    }
}

Suggested Enhancement

Spring gRPC should provide automatic baggage propagation from gRPC metadata, similar to the existing HTTP header propagation in Spring Boot. This could be:

1. Automatically enabled when management.tracing.baggage.remote-fields is configured
2. Integrated with Spring Boot's baggage configuration to avoid duplication
3. Support both baggage propagation and span tagging as configured

Environment

• Spring Boot: 4.0.2
• Spring gRPC: 1.0.2
• OpenTelemetry: (via spring-boot-starter-opentelemetry)

Benefits

• Consistent behavior between HTTP and gRPC for observability
• Reduced boilerplate code for gRPC services
• Better out-of-the-box tracing experience for gRPC applications
• Alignment with Spring Boot's observability conventions

Thank you for considering this enhancement!