feat(otlp-transformer): add subpath exports for trace, logs, and metrics

[overbalance]

What problem is this solving?

Browser and Edge Runtime consumers importing trace or log serializers inadvertently bundle @opentelemetry/sdk-metrics due to the barrel export. sdk-metrics uses setImmediate, which is unavailable in Edge Runtime, causing build warnings.

Tree-shaking cannot solve this because sdk-metrics has runtime dependencies that bundlers cannot statically eliminate. The import graph pulls in code that references setImmediate regardless of what you actually use.

Short description of changes

• Add subpath exports to package.json for signal-specific imports:
  • @opentelemetry/otlp-transformer/trace - trace serializers only
  • @opentelemetry/otlp-transformer/logs - log serializers only
  • @opentelemetry/otlp-transformer/metrics - metrics serializers only
• Export JsonTraceSerializer, ProtobufTraceSerializer from ./trace
• Export JsonLogsSerializer, ProtobufLogsSerializer from ./logs
• Export JsonMetricsSerializer, ProtobufMetricsSerializer from ./metrics

This follows existing precedent in @opentelemetry/otlp-exporter-base, which has ./node-http and ./browser-http subpath exports.

This is non-breaking. The main entry point continues to export everything.

Usage:

// Before: pulls in sdk-metrics
import { JsonTraceSerializer } from '@opentelemetry/otlp-transformer';

// After: only trace code bundled
import { JsonTraceSerializer } from '@opentelemetry/otlp-transformer/trace';

Testing

• Existing tests pass
• Verified Edge Runtime builds without warnings using Next.js 15 middleware
• Verified subpath imports resolve correctly for both ESM and CJS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.41%. Comparing base (0a1a631) to head (2d45fb0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6199   +/-   ##
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  Coverage   95.41%   95.41%           
=======================================
  Files         317      317           
  Lines        9460     9460           
  Branches     2199     2199           
=======================================
  Hits         9026     9026           
  Misses        434      434           

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[pichlermarc]

Hmm, looking at the code, I'd say the setImmediate call there is not strictly necessary.
The common code-path for the ConsoleMetricExporter runs synchronously too without any issues.

WDYT about removing the call to setImmediate() and instead invoking the callback directly to solve the problem?