feat(tracing): add bearer token authentication for OTLP exporters

Add BearerTokenFunc to support dynamic bearer token authentication
for OTLP gRPC exporters. Tokens are injected via gRPC PerRPCCredentials
on each export request.

- Add BearerTokenFunc type and Config field in tracerconfig
- Implement bearerCredentials (gRPC) and bearerRoundTripper (HTTP)
- Wire bearer auth into all three gRPC exporter creation functions
- Add token verification before flushing buffered logs
- Fix race condition in buffering exporter initialization

Note: HTTP exporters don't support dynamic bearer tokens due to
OpenTelemetry SDK limitations (no WithHTTPClient option). Use gRPC
protocol for dynamic tokens.

tracing/tracing.go

@@ -112,6 +112,10 @@ func Start(ctx context.Context, spanName string, opts ...trace.SpanStartOption)
 // This maintains backward compatibility for existing code.
 type GetClientCertificate = tracerconfig.GetClientCertificate
 
+// BearerTokenFunc is an alias for the type defined in tracerconfig.
+// It retrieves a bearer token for OTLP authentication.
+type BearerTokenFunc = tracerconfig.BearerTokenFunc
+
 // TracerConfig provides configuration options for OpenTelemetry tracing setup.
 // It supplements standard OpenTelemetry environment variables with additional
 // NTP Pool-specific configuration including TLS settings for secure OTLP export.
@@ -123,6 +127,7 @@ type TracerConfig struct {
 
 	CertificateProvider GetClientCertificate // Client certificate provider for mutual TLS
 	RootCAs             *x509.CertPool       // CA certificate pool for server verification
+	BearerTokenFunc     BearerTokenFunc      // Token provider for bearer authentication
 }
 
 // InitTracer initializes the OpenTelemetry SDK with the provided configuration.
@@ -160,6 +165,7 @@ func SetupSDK(ctx context.Context, cfg *TracerConfig) (shutdown TpShutdownFunc,
 		EndpointURL:         cfg.EndpointURL,
 		CertificateProvider: cfg.CertificateProvider,
 		RootCAs:             cfg.RootCAs,
+		BearerTokenFunc:     cfg.BearerTokenFunc,
 	}
 	tracerconfig.Store(ctx, bridgeConfig, createOTLPLogExporter, createOTLPMetricExporter, createOTLPTraceExporter)