fix(metrics): Set Metrics Interface backend to Kafka in self-hosted

[ayirr7]

The default behavior is for this backend to be pointing at the Snuba backend, which is incorrect. We want to be using the backend which directly produces the metrics onto the indexer (in the generic metrics platform), which is the Kafka-based backend.

[hubertdeng123]

This cannot go in until after getsentry/self-hosted#2355 goes in. I plan to merge the self-hosted PR this week

[codecov]

Codecov Report

Merging #55374 (d1a068b) into master (e78c7b4) will increase coverage by 0.00%.
Report is 4 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #55374   +/-   ##
=======================================
  Coverage   80.01%   80.01%           
=======================================
  Files        5042     5042           
  Lines      214970   214970           
  Branches    36501    36501           
=======================================
+ Hits       171998   172004    +6     
+ Misses      37686    37683    -3     
+ Partials     5286     5283    -3     

see 3 files with indirect coverage changes

[getsantry]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you remove the label Waiting for: Community, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀