metrics: allow sending only initial TTFB metrics

src/pipecat/frames/frames.py

@@ -189,6 +189,7 @@ class StartFrame(SystemFrame):
     """This is the first frame that should be pushed down a pipeline."""
     allow_interruptions: bool = False
     enable_metrics: bool = False
+    report_only_initial_ttfb: bool = False
 
 
 @dataclass