opentelemetry input plugin response does not match OTLP/HTTP specification

[ajschmidt8]

Bug Report

Describe the bug
The HTTP response from the opentelemetry input plugin does not match the OTLP/HTTP specification.

In particular, it doesn't include:

• the appropriate serialized Protobuf message (full success, partial success, or failure)
• a Content-Type header

These requirements can be viewed in the OTLP/HTTP response specification here: https://opentelemetry.io/docs/specs/otlp/#otlphttp-response

To Reproduce

• Run Fluent Bit version 3.2 with the opentelemetry input plugin configured
• Use protocurl to send an example trace to Fluent Bit, e.g.:

  protocurl \
   -i opentelemetry.proto.collector.trace.v1.ExportTraceServiceRequest \
   -o opentelemetry.proto.collector.trace.v1.ExportTraceServiceResponse \
   -u http://localhost:4318/v1/traces \
   -d @trace.json -I op -D

• Notice the empty response body and missing headers in the screenshot section below

Expected behavior

The opentelemetry input plugin should return a response with headers and a body that matches the OTLP/HTTP specification.

Screenshots

This screenshot shows the missing headers and body content from the Fluent Bit opentelemetry input plugin response:

This is in contrast with other OTLP/HTTP implementations, like Grafana Tempo, which issue a response shown in the following screenshot:

Your Environment

• Version used: 3.2
• Configuration:

  [SERVICE]
      Flush                1
      Log_level            debug
  
  [INPUT]
      name    opentelemetry
      listen  0.0.0.0
      port    4318
  
  [OUTPUT]
      Name                 opentelemetry
      Match                *
      Host                 tempo
      Port                 4318
      Log_response_payload True

• Operating System and version: Ubuntu 22.04
• Filters and plugins: opentelemetry input plugin

Additional context

This issue has caused problems some the otel-cli client that we use. Specifically, the lines below throw an error due to the missing Content-Type headers.

• https://github.com/equinix-labs/otel-cli/blob/3a5648dd799fcb430275a1c139e01b30f4ba22ef/otlpclient/otlp_client_http.go#L91-L97

cc: @msarahan