out_opentelemetry: add gzip and zstd compression for grpc

[flobz]

Add gzip compression for otlp grpc output.

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Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

Summary by CodeRabbit

• Bug Fixes
  • Fixed gRPC-over-HTTP/2 payload compression for gzip and zstd.
  • Corrected compression signaling and encoding headers for compressed requests.
  • Added fallback to uncompressed payloads when compression fails.
  • Improved handling of encoding-header failures by retrying affected requests.
  • Improved gRPC response reporting by displaying status codes and messages instead of response bodies.

[flobz]

Fluent config:

[SERVICE]
    Flush     1
    Daemon    off
    Log_Level debug


[INPUT]
    Name                 node_exporter_metrics
    Tag                  system_metrics
    Scrape_interval      10

[OUTPUT]
    Name                 opentelemetry
    Match                *
    Host                 127.0.0.1
    Port                 4317
    grpc                 true
    http2               force
    Log_response_payload True
    Tls                   Off
   Compress       gzip

Open telemetry collector config:

receivers:
  otlp:
    protocols:
     grpc:
        endpoint: 0.0.0.0:4317
       
exporters:
      debug:
        verbosity: detailed
service:
  pipelines:
    metrics:
        receivers: [otlp]
        exporters: [debug]
  telemetry:
    logs:
     level: debug
     development: true

Valgrind report:

==4060852== 
==4060852== HEAP SUMMARY:
==4060852==     in use at exit: 0 bytes in 0 blocks
==4060852==   total heap usage: 45,211 allocs, 45,211 frees, 32,868,495 bytes allocated
==4060852== 
==4060852== All heap blocks were freed -- no leaks are possible
==4060852== 
==4060852== For lists of detected and suppressed errors, rerun with: -s

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• plugins/out_opentelemetry/opentelemetry.c

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📝 Walkthrough

Walkthrough

The OpenTelemetry gRPC-over-HTTP/2 path compresses payloads with gzip or zstd, frames compressed data, handles temporary buffers and header failures, and logs gRPC response status metadata.

Changes

gRPC transport updates

Layer / File(s)	Summary
Compress and frame gRPC payloads plugins/out_opentelemetry/opentelemetry.c	The request path compresses payloads with gzip or zstd. It sets the gRPC compression flag and encoding headers. Failure paths release compressed buffers and propagate grpc-encoding header failures.
Log gRPC response status plugins/out_opentelemetry/opentelemetry.c	Successful application/grpc responses now log grpc-status and grpc-message instead of the response body.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant opentelemetry_post
  participant gzip_zstd_compressor
  participant grpc_body
  participant HTTP2_request
  opentelemetry_post->>gzip_zstd_compressor: compress payload
  gzip_zstd_compressor-->>opentelemetry_post: return final_body and final_body_len
  opentelemetry_post->>grpc_body: add compression flag and payload
  opentelemetry_post->>HTTP2_request: send request with encoding headers

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Merge Risk: ⚪ Minimal · up to 72f2f

No merge-blocking risk remains from the reviewed changes.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding gzip and zstd compression support for gRPC in the OpenTelemetry output.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 3

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@plugins/out_opentelemetry/opentelemetry.c`:
- Around line 722-734: Update the gRPC logging block around response
content-type detection to guard response->content_type before calling
strncasecmp, retrieve grpc-status and grpc-message from trailers before headers,
and pass non-NULL fallback strings to the %s log arguments. Add a regression
test covering trailer-only grpc-status with no grpc-message.
- Around line 570-574: Check the return value of flb_http_request_set_header for
the grpc-encoding header in the compressed-frame request path; on failure,
destroy the request, free final_body, and return FLB_RETRY, matching the
existing TE header failure handling.
- Around line 481-484: Update the compression failure branch in the
OpenTelemetry framing path to fall back to the original payload by assigning
final_body and final_body_len from the uncompressed input when
flb_gzip_compress() or flb_zstd_compress() returns an error. Keep
compression_algorithm disabled and preserve the existing error logging.

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