Standardize Gitaly log timestamps

Problem to solve

Under &2575 we are standardizing all log timestamps to ISO 8601.3 format (ISO 8601 with millisecond precision, like "2018-05-23T11:14:50.456Z"). Currently log entries for Gitaly use ordinary ISO 8601 format for timestamps (without second precision, like "2018-05-23T11:14:50Z").

Proposal

Add millisecond precision to Gitaly log timestamps so they are in ISO 8601.3 format.

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