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Resolve "Track the number of new Redis connections per transaction"

username-removed-443319 requested to merge 18451-track-new-redis-connections into master

What does this MR do?

Add a new metric counter, new_redis_connections, that contains the number of calls to Redis::Client#connect in the current transaction.

Are there points in the code the reviewer needs to double check?

Not sure. I tested this in kind of a brute-force way:

  1. Add a debugger in the monkey-patched connect method.
  2. With metrics enabled, start the app and load a page.
  3. The first Redis connection is created by Rack::Attack and isn't in a transaction, but still works fine.
  4. The second Redis connection is within a transaction (the page load), and increments the counter.
  5. If I reload the page, neither debugger is hit.
  6. If I use a Redis client and do CLIENT KILL on my two existing clients, then reload the page, I get 3 and 4 again.
  7. If I disable metrics collection, the debugger never gets hit.

Why was this MR needed?

We may have a Redis connection leak somewhere, so adding metrics will let us track this.

What are the relevant issue numbers?

Closes #18451 (closed).

Screenshots (if relevant)

Hahaha nope, not relevant.

Does this MR meet the acceptance criteria?

cc @yorickpeterse

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