Remove processing metrics (ProcessingMetricsData)

Processing metrics were an early addition that predated a clear
understanding of what timing measurements matter in real-time pipelines.
They were inconsistently implemented across services, often broken, and
overlapped with the better-defined TTFB metric.

- Remove ProcessingMetricsData class and all start/stop_processing_metrics
  methods from FrameProcessorMetrics, FrameProcessor, and SentryMetrics
- Remove all processing metrics calls from 31 service files (LLM, TTS,
  STT, image, vision, realtime)
- Clean up empty _start_metrics() stubs left in STT services
- Remove processing metrics handling from RTVI, metrics log observer,
  pipeline task initial metrics, and strands agents framework
- Update tests and examples

Remaining metrics (TTFB, LLM token usage, TTS character usage, text
aggregation time) are well-defined and consistently implemented.

examples/foundational/06-listen-and-respond.py

@@ -13,7 +13,6 @@ from pipecat.audio.vad.silero import SileroVADAnalyzer
 from pipecat.frames.frames import Frame, LLMRunFrame, MetricsFrame
 from pipecat.metrics.metrics import (
     LLMUsageMetricsData,
-    ProcessingMetricsData,
     TTFBMetricsData,
     TTSUsageMetricsData,
 )
@@ -46,8 +45,6 @@ class MetricsLogger(FrameProcessor):
             for d in frame.data:
                 if isinstance(d, TTFBMetricsData):
                     print(f"!!! MetricsFrame: {frame}, ttfb: {d.value}")
-                elif isinstance(d, ProcessingMetricsData):
-                    print(f"!!! MetricsFrame: {frame}, processing: {d.value}")
                 elif isinstance(d, LLMUsageMetricsData):
                     tokens = d.value
                     print(