Add BearerTokenFunc to support dynamic bearer token authentication
for OTLP exporters. Tokens are injected per-request via gRPC
PerRPCCredentials and HTTP custom RoundTripper.
- Add BearerTokenFunc type and Config field in tracerconfig
- Implement bearerCredentials (gRPC) and bearerRoundTripper (HTTP)
- Wire bearer auth into all exporter creation functions
- Add getHTTPClient helper for DRY HTTP client configuration
- Upgrade OpenTelemetry SDK to v1.39.0 for WithHTTPClient support
Remove sync.Once reset that caused "unlock of unlocked mutex" panic.
Redesign initialization to use only checkReadiness goroutine for
retry attempts, eliminating race condition while preserving retry
functionality for TLS/tracing setup delays.
Add buffering exporter to queue OTLP logs until tracing is configured.
Support TLS configuration for OpenTelemetry log export with client
certificate authentication. Improve logfmt formatting and tracing setup.
