The optional label API was added in 4c78a974 and xds_cluster_impl was
plumbed in 077dcbf9.
From gRFC A78:
> ### Optional xDS Locality Label
>
> When xDS is used, it is desirable for some metrics to include an optional
> label indicating which xDS locality the metrics are associated with.
> We want to provide this optional label for the metrics in both the
> existing per-call metrics defined in [A66] and in the new metrics for
> the WRR LB policy, described below.
>
> If locality information is available, the value of this label will be of
> the form `{region="${REGION}", zone="${ZONE}", sub_zone="${SUB_ZONE}"}`,
> where `${REGION}`, `${ZONE}`, and `${SUB_ZONE}` are replaced with the
> actual values. If no locality information is available, the label will
> be set to the empty string.
>
> #### Per-Call Metrics
>
> To support the locality label in the per-call metrics, we will provide
> a mechanism for LB picker to add optional labels to the call attempt
> tracer. We will then use this mechanism in the `xds_cluster_impl`
> policy's picker to set the locality label. ...
>
> This label will be available on the following per-call metrics:
> - `grpc.client.attempt.duration`
> - `grpc.client.attempt.sent_total_compressed_message_size`
> - `grpc.client.attempt.rcvd_total_compressed_message_size`
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