[Backport 5.2] Fix data access race condition in gRPC retry logic

[sourcegraph-release-bot]

		callOpts := reuseOrNewWithCallOptions(intOpts, retryOpts) // callOpts are recycled here if no call opts given

		doTrace := makeTracingCallback(parentCtx, logger, service, method)
		originalCallback := callOpts.onRetryCallback
		callOpts.onRetryCallback = func(ctx context.Context, attempt uint, err error) { // and is overwritten here, potentially by multiple callers at the same time.
			doTrace(attempt, err)

			if originalCallback != nil {
				originalCallback(ctx, attempt, err)
			}
		}

Test plan

go test ./cmd/gitserver/internal/integration_tests -race -count=5 consistently fails before this change on branch es/grpc-cleanup where we switched the integration test suite from HTTP to gRPC (the default option). With this patch applied, it no longer fails.
Backport adb95ea from #59487

[ggilmore]

cc @sourcegraph/release-guild Please stamp, thanks!

[sourcegraph-bot]

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