[chore][mdatagen] Add note about the supported types of telemetry met…

…ric attributes (#10997)

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#### Description
The context can be found
[here.](#10801 (comment))
Only a subset of attribute types are supported for `telemetry` metrics
vs. regular `metrics`. There's no instrumentation equivalent for
attributes of the type `bytes`, `slice`, and `map`.

cmd/mdatagen/metadata-schema.yaml

@@ -168,4 +168,5 @@ telemetry:
         # Bucket boundaries are only available to set for histogram metrics.
         bucket_boundaries: [double]
       # Optional: array of attributes that were defined in the attributes section that are emitted by this metric.
+      # Note: Only the following attribute types are supported: <string|int|double|bool>
       attributes: [string]