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

## 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 ...