gitaly: Compute initial_backoff for Gitaly client
What does this MR do and why?
A previous merge request allowed clients to configure the max_backoff and max_attempts of the retry policy of the Gitaly client.
See here:
But it turns out that the gRPC library caps the max attempts to 5, even if a user configure a much higher value.
That limitation defeats a bit the purpose of exposing those new fields to outside configuration. With the current hardcoded initial_backoff of 0.4 seconds, the maximum backoff time is 12.8 seconds, even if a clients set max_backoff to 20 seconds.
This commit introduces computing the initial backoff time from the max_backoff and max_attempts, essentially making max_backoff the maximum amount of time the client will keep on retrying.
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