pw_metric: Add comment and fix typo in proto
pw_metric: Add comment and fix typo in proto

Change-Id: Ic1f5fa8174e73306cd6ab3130e42ca12becbd0f1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/20180
Reviewed-by: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed