Git service latency spikes

As @engwan noticed in production#6230 (comment 822214466), we have periodic bursts of slow Rails requests in ServiceGit . The slowness seems to be due to redis-cache calls.

This issue is for investigating these bursts.

Log example

Conclusion 2021-02-01

We should upgrade to Redis 6.2.0 or newer so that we get better maxmemory eviction behavior from Redis. #1556 (comment 828201892)