Tag Tokenserver's token_verification metric with BrowserID vs OAuth

[data-sync-user]

Tokenserver emits a syncstorage.tokenserver.token_verification metric that reports the amount of time in milliseconds it took to verify a token (be it an OAuth token or BrowserID assertion). OAuth tokens and BrowserID assertions are verified very differently, with OAuth tokens verified locally and BrowserID assertions requiring a request to FxA, which makes the performance of each of these operations rather different. As it exists today, the metric only provides timing statistics about OAuth and BrowserID verification in aggregate. It would be beneficial for characterizing Tokenserver’s performance under production conditions to tag this metric so we have visibility into token verification performance for BrowserID and OAuth separately.

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