feat(instrumentation/asgi): add target to metrics

This PR adds the target information for metrics reported by instrumentation/asgi.

Unfortunately, there's no ASGI standard to reliably get this information, and I was only able to get it for FastAPI.

I also tried to get the info with Sanic and Starlette (encode/starlette#685), but there's nothing in the scope allowing to recreate the route.

Besides the included unit tests, the logic was tested using the following app:

```python
import io
import fastapi

app = fastapi.FastAPI()

def dump_scope(scope):
    b = io.StringIO()
    print(scope, file=b)
    return b.getvalue()

@app.get("/test/{id}")
def test(id: str, req: fastapi.Request):
    print(req.scope)
    return {"target": _collect_target_attribute(req.scope), "scope": dump_scope(req.scope)}

sub_app = fastapi.FastAPI()

@sub_app.get("/test/{id}")
def sub_test(id: str, req: fastapi.Request):
    print(req.scope)
    return {"target": _collect_target_attribute(req.scope), "scope": dump_scope(req.scope)}

app.mount("/sub", sub_app)
```

Partially fixes open-telemetry#1116

Note to reviewers: I tried to touch as less as possible, so that we don;t require a refactor before this change. However, we could consider changing `collect_request_attributes` so that it returns both a trace attributes and a metrics attributes.
Wihout that change we cannot add the `HTTP_TARGET` attribute to the list of metric atttributes, because it will be present but with high cardinality.

instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py

@@ -365,6 +365,34 @@ def get_default_span_details(scope: dict) -> Tuple[str, dict]:
     return span_name, {}
 
 
+def _collect_target_attribute(
+    scope: typing.Dict[str, typing.Any]
+) -> typing.Optional[str]:
+    """
+    Returns the target path as defined by the Semantic Conventions.
+
+    This value is suitable to use in metrics as it should replace concrete
+    values with a parameterized name. Example: /api/users/{user_id}
+
+    Refer to the specification
+    https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/semantic_conventions/http-metrics.md#parameterized-attributes
+
+    Note: this function requires specific code for each framework, as there's no
+    standard attribute to use.
+    """
+    # FastAPI
+    root_path = scope.get("root_path", "")
+
+    route = scope.get("route")
+    if not route:
+        return None
+    path_format = getattr(route, "path_format", None)
+    if path_format:
+        return f"{root_path}{path_format}"
+
+    return None
+
+
 class OpenTelemetryMiddleware:
     """The ASGI application middleware.
 
@@ -448,6 +476,12 @@ async def __call__(self, scope, receive, send):
             attributes
         )
         duration_attrs = _parse_duration_attrs(attributes)
+
+        target = _collect_target_attribute(scope)
+        if target:
+            active_requests_count_attrs[SpanAttributes.HTTP_TARGET] = target
+            duration_attrs[SpanAttributes.HTTP_TARGET] = target
+
         if scope["type"] == "http":
             self.active_requests_counter.add(1, active_requests_count_attrs)
         try:

instrumentation/opentelemetry-instrumentation-asgi/tests/test_asgi_middleware.py

@@ -612,6 +612,37 @@ def test_basic_metric_success(self):
                             )
                             self.assertEqual(point.value, 0)
 
+    def test_metric_target_attribute(self):
+        expected_target = "/api/user/{id}"
+
+        class TestRoute:
+            path_format = expected_target
+
+        self.scope["route"] = TestRoute()
+        app = otel_asgi.OpenTelemetryMiddleware(simple_asgi)
+        self.seed_app(app)
+        self.send_default_request()
+
+        metrics_list = self.memory_metrics_reader.get_metrics_data()
+        assertions = 0
+        for resource_metric in metrics_list.resource_metrics:
+            for scope_metrics in resource_metric.scope_metrics:
+                for metric in scope_metrics.metrics:
+                    for point in metric.data.data_points:
+                        if isinstance(point, HistogramDataPoint):
+                            self.assertEqual(
+                                point.attributes["http.target"],
+                                expected_target,
+                            )
+                            assertions += 1
+                        elif isinstance(point, NumberDataPoint):
+                            self.assertEqual(
+                                point.attributes["http.target"],
+                                expected_target,
+                            )
+                            assertions += 1
+        self.assertEqual(assertions, 2)
+
     def test_no_metric_for_websockets(self):
         self.scope = {
             "type": "websocket",
@@ -705,6 +736,36 @@ def test_credential_removal(self):
             attrs[SpanAttributes.HTTP_URL], "http://httpbin.org/status/200"
         )
 
+    def test_collect_target_attribute_missing(self):
+        self.assertIsNone(otel_asgi._collect_target_attribute(self.scope))
+
+    def test_collect_target_attribute_fastapi(self):
+        class TestRoute:
+            path_format = "/api/users/{user_id}"
+
+        self.scope["route"] = TestRoute()
+        self.assertEqual(
+            otel_asgi._collect_target_attribute(self.scope),
+            "/api/users/{user_id}",
+        )
+
+    def test_collect_target_attribute_fastapi_mounted(self):
+        class TestRoute:
+            path_format = "/users/{user_id}"
+
+        self.scope["route"] = TestRoute()
+        self.scope["root_path"] = "/api/v2"
+        self.assertEqual(
+            otel_asgi._collect_target_attribute(self.scope),
+            "/api/v2/users/{user_id}",
+        )
+
+    def test_collect_target_attribute_fastapi_starlette_invalid(self):
+        self.scope["route"] = object()
+        self.assertIsNone(
+            otel_asgi._collect_target_attribute(self.scope), None
+        )
+
 
 class TestWrappedApplication(AsgiTestBase):
     def test_mark_span_internal_in_presence_of_span_from_other_framework(self):