feat(observability): wire unused MyAudio Prometheus metrics (#2232)

* feat(observability): wire unused MyAudio Prometheus metrics to call sites

Connect 4 previously-defined but uncalled metrics to their logical
call sites in the audio processing pipeline:

- RecordAudioInferenceDuration: records BirdNET inference elapsed time
- RecordBirdNETResults: records detection result count per inference
- RecordAudioQueueOperation: tracks enqueue success/drop on results queue
- RecordAudioDataSize: tracks cumulative bytes written to analysis buffer

* fix(observability): measure only Predict() latency for inference duration metric

Separate inference timing from end-to-end processing time so
RecordAudioInferenceDuration accurately reflects only bn.Predict()
latency. Also record inference duration on failed predictions.

---------

Co-authored-by: tphakala <tphakala@users.noreply.github.com>

Author: tphakala

internal/myaudio/analysis_buffer.go

@@ -387,6 +387,7 @@ func WriteToAnalysisBuffer(sourceID string, data []byte) error {
 				m.RecordBufferWrite("analysis", sourceID, "success")
 				m.RecordBufferWriteDuration("analysis", sourceID, duration)
 				m.RecordBufferWriteBytes("analysis", sourceID, n)
+				m.RecordAudioDataSize(sourceID, n)
 			}
 
 			if n < len(data) {

internal/myaudio/process.go

@@ -152,21 +152,36 @@ func ProcessData(bn *birdnet.BirdNET, data []byte, startTime time.Time, source s
 	}
 
 	// run BirdNET inference
+	inferenceStart := time.Now()
 	results, err := bn.Predict(sampleData)
+	inferenceDuration := time.Since(inferenceStart)
 
 	// Return float32 buffer to pool after prediction
 	// This is safe because Predict copies the data to the input tensor
 	if conf.BitDepth == 16 && len(sampleData) > 0 && len(sampleData[0]) == Float32BufferSize {
 		ReturnFloat32Buffer(sampleData[0])
 	}
 
+	// Record inference duration metric (always, even on error)
+	processMetricsMutex.RLock()
+	pm := processMetrics
+	processMetricsMutex.RUnlock()
+	if pm != nil {
+		pm.RecordAudioInferenceDuration(source, inferenceDuration.Seconds())
+	}
+
 	if err != nil {
 		return fmt.Errorf("error predicting species: %w", err)
 	}
 
-	// get elapsed time
+	// get elapsed time (includes conversion + inference for overrun check)
 	elapsedTime := time.Since(predictStart)
 
+	// Record result count metric
+	if pm != nil {
+		pm.RecordBirdNETResults(source, len(results))
+	}
+
 	// DEBUG print all BirdNET results
 	if conf.Setting().BirdNET.Debug {
 		debugThreshold := float32(0) // set to 0 for now, maybe add a config option later
@@ -264,11 +279,15 @@ func ProcessData(bn *birdnet.BirdNET, data []byte, startTime time.Time, source s
 	// Note: No copy needed - ownership transfers to the queue consumer
 	select {
 	case birdnet.ResultsQueue <- resultsMessage:
-		// Results enqueued successfully
+		if pm != nil {
+			pm.RecordAudioQueueOperation(source, "enqueue", "success")
+		}
 	default:
 		log.Error("results queue is full",
 			logger.String("source", source))
-		// Queue is full
+		if pm != nil {
+			pm.RecordAudioQueueOperation(source, "enqueue", "dropped")
+		}
 	}
 	return nil
 }