feat: propagate OTEL env var attributes as default log fields

[dnjg]

Summary

• Reads OTEL_SERVICE_NAME and OTEL_RESOURCE_ATTRIBUTES at logger init and injects them as default zap fields, so logs carry the same resource context as traces and metrics.
• Fixes stale ldflags path in Makefile and updates README with corrected examples and span/meter usage docs.

Test plan

• Unit tests for otelEnvFields() covering all cases (no vars, one set, both, malformed, values with =)
• Integration test verifying fields appear in JSON log output
• All existing tests pass (go test ./pkg/logger/...)
• Linter passes (make lint)

[dnjg]

The idea here is to have the same attributes available one logs, traces, and metrics.

Also this way in grafana alloy we can just read these fields rather than inferring the labels from things like the log group, which doesn't seem to work quite right for ecs

[dnjg]

Ill leave this one up till @vik-nullify gets a chance to review it, in case this is a bad idea with alloy.

only thing I can think right now is it'll add a bit of size overhead to each log

[vik-nullify]

It does, lets see how useful it is and think about pruning the logs with just right labels as a final pass after this release