Praefect: Use millisecond precision for JSON logs time field

Problem to solve

A customer reported that times in Praefect logs are not standardized. Some items have millisecond precision (thanks to !1887 (merged)), but others don't.

From the customer:

We are trying to ingest logs on Datalog and are facing dates on logs that aren't standardized.  Example on the same file (/var/log/gitlab/praefect/current):

{"gitaly":15927,"level":"warning","msg":"forwarding signal","signal":23,"time":"2020-12-09T15:24:49Z","wrapper":15937}
{"correlation_id":"M2qIAurFTg7","grpc.code":"OK","grpc.meta.auth_version":"v2","grpc.meta.client_name":"gitlab-web","grpc.meta.deadline_type":"regular","grpc.method":"FindCommit","grpc.request.deadline":"2020-12-09T14:39:10Z","grpc.request.fullMethod":"/gitaly.CommitService/FindCommit","grpc.service":"gitaly.CommitService","grpc.start_time":"2020-12-09T14:38:41Z","grpc.time_ms":4.497,"level":"info","msg":"finished streaming call with code OK","peer.address":"REDACTED:11670","pid":15927,"relative_path":"@hashed/e7/f6/REDACTED.git","span.kind":"server","system":"grpc","time":"2020-12-09T14:38:41.010Z","virtual_storage":"distributed"}

Notice that the first time does not have millisecond precision, but the last time does.

Proposal

Praefect log entries should have times with millisecond precision (ISO 8601.3).