[chore][mdatagen] Update schema with information on async telemetry m…

…etrics (#10998)

#### Description
The `async` option was recently added to telemetry metrics, and I
believe this should be documented in the metadata schema. I've added a
short description about what it means, and its purpose.

#### Link to tracking issue
Resolves
#10990

cmd/mdatagen/metadata-schema.yaml

@@ -158,6 +158,9 @@ telemetry:
       unit:
       # Required: metric type with its settings.
       <sum|gauge|histogram>:
+        # Optional: Whether this metric is asynchronous. If async, a mechanism is required to be able to
+        # pass in options to the callbacks that are called when the metric is observed.
+        async: bool
         # Required: type of number data point values.
         value_type: <int|double>
         # Required for sum metric: whether the metric is monotonic (no negative delta values).