This adds a metric RPC service, which exposes a metric tree. The user
selects which metrics to expose. The metric tree is sent via streaming
RPC to enable streaming potentially large sets of metrics.
Change-Id: Ib308995b8f7f91ce439ca42fe298f802d4bdf7e3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/16003
Commit-Queue: Keir Mierle <keir@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
