fix(tracing): address audit findings

Default to TLS export with Insecure/TLSConfig/Headers options, parent-based
ratio sampling (default 0.1), and no query string or Host in span attributes.
Name spans by route template, record status and panics (re-raised), guard the
tracer with atomic.Pointer, reject double init, add init timeout and attribute
length limit, and move to semconv v1.26.0. Add tracing.insecure and
tracing.sample_rate config keys and tests.

pkg/config/config.go

@@ -91,6 +91,12 @@ type TracingConfig struct {
 	ServiceName    string `mapstructure:"service_name"`
 	ServiceVersion string `mapstructure:"service_version"`
 	Endpoint       string `mapstructure:"endpoint"`
+	// Insecure exports traces over plaintext gRPC (default false: TLS).
+	Insecure bool `mapstructure:"insecure"`
+	// SampleRate is the fraction of root traces sampled; 0 selects the default (0.1).
+	SampleRate float64 `mapstructure:"sample_rate"`
+	// Headers are sent with every OTLP export request (e.g. auth tokens).
+	Headers map[string]string `mapstructure:"headers"`
 }
 
 // CacheConfig holds cache provider configuration