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Provides API support for specifying an optional user-defined key along
with the metadata name. This can be used to distinguish samples based
on additional state beyond just the name -- e.g. execution in service
of different tabs within the same renderer process.

Uploading of the key state with the samples will be implemented in a
following CL.

Bug: 976864
Change-Id: Ib924be48a816e65e83976c34e27efea19da61925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891112Reviewed-by: Charlie Andrews <charliea@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711265}