add and set alert_end_time

[Abigail-Yates]

• Add alert_end_time as a prometheus metric. It will be a unix timestamp.
• Set alert_end_time when session is created (if one exists and is further in the future than the current alert_end_time)
• Update alert_end_time when session end time is updated

(also updated one log warning statement in the create silence endpoint to make it consistent with the others)

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.08%. Comparing base (408efc8) to head (69cdc18).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #806      +/-   ##
==========================================
- Coverage   52.12%   52.08%   -0.05%     
==========================================
  Files          96       96              
  Lines       10402    10414      +12     
  Branches     1377     1381       +4     
==========================================
+ Hits         5422     5424       +2     
- Misses       4696     4706      +10     
  Partials      284      284              

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[stephen-riggs]

All looks good to me