With d5e4aa25 (Configure retries for read-only Gitaly RPCs,
2023-10-11) we configured the Gitaly client to send up to two retries
on read-only RPCs, waiting no more than 750ms in total.

After rolling out the new Gitaly upgrade process to canary, we have
observed that Gitaly restarts may take up to two seconds to begin
accepting traffic[0], more than the grace period configured for retries.

Expand the retry policy to allow up to three retries with a starting
delay of 0.4 seconds and a maximum backoff of 1.4 seconds. This gives
us a total grace period of 2.4 seconds, which is still well within the
default deadline of 10 seconds.

[0] gitaly#5744 (comment 1736838569)