[telemetry] - Fix KeyError when traversing metadata_records.
[telemetry] - Fix KeyError when traversing metadata_records.

AFAIK self._raw_data is the raw json, which in this case would look something like
{
  'traceevents': {...},
  'metadata': {...},
}

It looks like the intention is to return the value of the metadata field, but instead we're trying to index the json with the value of the field.

BUG=515206

Review URL: https://codereview.chromium.org/1258913003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#340981}

CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend