Track response start duration

This commit adds a feature to track the latency excluding
streaming duration.

src/prometheus_fastapi_instrumentator/metrics.py

@@ -27,6 +27,7 @@ def __init__(
         modified_handler: str,
         modified_status: str,
         modified_duration: float,
+        modified_duration_without_streaming: float,
     ):
         """Creates Info object that is used for instrumentation functions.
 
@@ -42,6 +43,8 @@ def __init__(
                 by instrumentator. For example grouping into `2xx`, `3xx` and so on.
             modified_duration (float): Latency representation after processing
                 by instrumentator. For example rounding of decimals. Seconds.
+            modified_duration_without_streaming (float): Latency between request arrival and response starts (i.e. first chunk duration).
+                Excluding the streaming duration.
         """
 
         self.request = request
@@ -50,6 +53,7 @@ def __init__(
         self.modified_handler = modified_handler
         self.modified_status = modified_status
         self.modified_duration = modified_duration
+        self.modified_duration_without_streaming = modified_duration_without_streaming
 
 
 def _build_label_attribute_names(
@@ -114,6 +118,7 @@ def latency(
     should_include_handler: bool = True,
     should_include_method: bool = True,
     should_include_status: bool = True,
+    should_exclude_streaming_duration: bool = False,
     buckets: Sequence[Union[float, str]] = Histogram.DEFAULT_BUCKETS,
     registry: CollectorRegistry = REGISTRY,
 ) -> Optional[Callable[[Info], None]]:
@@ -184,15 +189,19 @@ def latency(
             )
 
         def instrumentation(info: Info) -> None:
+            duration = info.modified_duration
+            if should_exclude_streaming_duration == True:
+                duration = info.modified_duration_without_streaming
+
             if label_names:
                 label_values = [
                     getattr(info, attribute_name)
                     for attribute_name in info_attribute_names
                 ]
 
-                METRIC.labels(*label_values).observe(info.modified_duration)
+                METRIC.labels(*label_values).observe(duration)
             else:
-                METRIC.observe(info.modified_duration)
+                METRIC.observe(duration)
 
         return instrumentation
     except ValueError as e:

src/prometheus_fastapi_instrumentator/middleware.py

@@ -140,6 +140,7 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         status_code = 500
         headers = []
         body = b""
+        response_start_time = None
 
         # Message body collected for handlers matching body_handlers patterns.
         if any(pattern.search(handler) for pattern in self.body_handlers):
@@ -158,9 +159,10 @@ async def send_wrapper(message: Message) -> None:
 
             async def send_wrapper(message: Message) -> None:
                 if message["type"] == "http.response.start":
-                    nonlocal status_code, headers
+                    nonlocal status_code, headers, response_start_time
                     headers = message["headers"]
                     status_code = message["status"]
+                    response_start_time = default_timer()
                 await send(message)
 
         try:
@@ -176,12 +178,17 @@ async def send_wrapper(message: Message) -> None:
 
             if not is_excluded:
                 duration = max(default_timer() - start_time, 0)
+                duration_without_streaming = 0
+
+                if response_start_time:
+                    duration_without_streaming = max(response_start_time - start_time, 0)
 
                 if self.should_instrument_requests_inprogress:
                     inprogress.dec()
 
                 if self.should_round_latency_decimals:
                     duration = round(duration, self.round_latency_decimals)
+                    duration_without_streaming = round(duration_without_streaming, self.round_latency_decimals)
 
                 if self.should_group_status_codes:
                     status = status[0] + "xx"
@@ -197,6 +204,7 @@ async def send_wrapper(message: Message) -> None:
                     modified_handler=handler,
                     modified_status=status,
                     modified_duration=duration,
+                    modified_duration_without_streaming=duration_without_streaming,
                 )
 
                 for instrumentation in self.instrumentations: