feat(otlp-transformer): add custom logs protobuf serializer

[pichlermarc]

Which problem is this PR solving?

Currently we run into warnings that CSPs are being violated by protobuf.js (#4987). Also, to convert to protobuf.js' format, we do have to go through an intermediate representation, which is costly in terms of allocations.

This PR moves toward fixing this by introducing a custom protobuf serializer that can be used instead.

Note: my previous PR #6228 had a simple dynamic growing approach. However, since there is a risk that memory behavior will change for the worse from the current protobuf.js-based implementation, which does a double-pass to determine the size of the final message, I opted to also implement a double-pass approach. The first pass determines the buffer size and the second pass actually writes to said buffer. This makes the approach slightly slower than what I proposed in #6228 (also slower than JSON serialization, but faster than the protobuf.js-based approach), but more predictable than the doubling the buffer size or always allocating 64KB, which may not be needed.

Towards fixing:

• use of eval is strongly discouraged #4987 (removing protobuf.js as dependency will get rid of the warning, and will enable use of protobuf exporters in the browser)
  • I also have a draft for a custom deserializer already, I'll open that one once this PR has merged. Since the response is fairly simple compared to the export request, it's relatively easy to parse.
• Stalled large POST requests in opentelemetry-js for web/browsers using exporter-trace-otlp-http #6274 (doing serialization ourselves will eventually allow us to split messages into chunks to avoid hitting client-side and server-side size limits)

Part of:

• Performance improvements #6100 (we can also continue improving with a custom implementation)

Supersedes #6228

Relevant benchmark results:

Old:

transform 512 logs (protobuf) x 502 ops/sec ±0.14% (96 runs sampled)
transform 512 logs (json) x 732 ops/sec ±0.19% (96 runs sampled)

New:

transform 512 logs (protobuf) x 650 ops/sec ±0.13% (97 runs sampled)
transform 512 logs (json) x 744 ops/sec ±0.17% (99 runs sampled)

Disclosure of AI use: I generated the initial prototype of this with GitHub Copilot and Claude Sonnet 4.5, but then applied a whole bunch of optimizations and changes to it to make it fit what we're trying to do here.

Short description of the changes

Type of change

• New feature

How Has This Been Tested?

• Added unit tests
• Existing unit tests
• Existing E2E tests
• Existing benchmarks

[codecov]

Codecov Report

❌ Patch coverage is 98.96907% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.61%. Comparing base (fcafab5) to head (67fc0c5).

Files with missing lines	Patch %	Lines
...ansformer/src/common/protobuf/common-serializer.ts	97.53%	2 Missing ⚠️
...p-transformer/src/logs/protobuf/logs-serializer.ts	98.19%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6390      +/-   ##
==========================================
+ Coverage   95.50%   95.61%   +0.11%     
==========================================
  Files         365      370       +5     
  Lines       11609    11992     +383     
  Branches     2677     2740      +63     
==========================================
+ Hits        11087    11466     +379     
- Misses        522      526       +4     

Files with missing lines	Coverage Δ
...mer/src/common/protobuf/protobuf-size-estimator.ts	100.00% <100.00%> (ø)
...transformer/src/common/protobuf/protobuf-writer.ts	100.00% <100.00%> (ø)
...ages/otlp-transformer/src/common/protobuf/utils.ts	100.00% <100.00%> (ø)
...ackages/otlp-transformer/src/logs/protobuf/logs.ts	100.00% <100.00%> (ø)
...ansformer/src/common/protobuf/common-serializer.ts	97.53% <97.53%> (ø)
...p-transformer/src/logs/protobuf/logs-serializer.ts	98.19% <98.19%> (ø)

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