[Tracing] Add metadata filter

Metadata fields are not currently filtered. This CL adds the
filter to strip out sensitive information is needed. It also
adds a whitelist of the metadata fields that are allowed to
be captured.

BUG=560444

Review URL: https://codereview.chromium.org/1502583003

Cr-Commit-Position: refs/heads/master@{#364778}
10 files changed