fix: Unstable PROMETHEUS_MULTIPROC_DIR

[khvn26]

In this PR, we change the following:

1. Use a default stable path for PROMETHEUS_MULTIPROC_DIR instead of relying on a random one offered by tempfile.mkdtemp to completely avoid new directories spam reported in temporary directories are not removed flagsmith#5990.
2. Move the cleanup to app startup. This ensures that the directory exists throughout Gunicorn's worker lifecycle.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.63%. Comparing base (00a973a) to head (6bf2ea8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
- Coverage   94.67%   94.63%   -0.05%     
==========================================
  Files          76       77       +1     
  Lines        2479     2459      -20     
==========================================
- Hits         2347     2327      -20     
  Misses        132      132              

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[emyller]

I don't have context yet, but I'm interested in possibly cleaning up temporary files on startup (just in case the previous run ended unexpectedly), and on shutdown (because it's polite to cleanup resources after use).

Unless there's reason I don't know, the better approach to manage temporary files should be via tempfile as long as we always cleanup.

[khvn26]

and on shutdown (because it's polite to cleanup resources after use).

I agree, but see below.

Unless there's reason I don't know, the better approach to manage temporary files should be via tempfile as long as we always cleanup.

tempfile.mkdtemp does not guarantee cleanup, and results in empty dir spam in some environments — see original issue Flagsmith/flagsmith#5990 for details.

temfile.TemporaryDirectory was supposed to work as we'd expect, but apparently cleans up too eagerly for Gunicorn's fork model (perhaps due to weakref being used) — see https://flagsmith.sentry.io/issues/6994120980/events/a31de60ca80e4ea08dda39669e9b02c5/. The issue is what this PR is fixing.

[emyller]

LGTM!