add in time to first token stat

changes stats to implement debug for histogram, update filter_context to
open ttft to stats endpoint and update stream_context to get time
between both of those.

crates/common/src/stats.rs

@@ -80,7 +80,7 @@ impl RecordingMetric for Gauge {}
 /// For offset deltas
 impl IncrementingMetric for Gauge {}
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub struct Histogram {
     id: u32,
 }