Adding missing instrumentation for APM.

[getvictor]

#19696

Checklist for submitter

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 63.16%. Comparing base (c93a28b) to head (7916c0d).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/fleet/serve.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23882   +/-   ##
=======================================
  Coverage   63.16%   63.16%           
=======================================
  Files        1560     1560           
  Lines      148412   148416    +4     
  Branches     3729     3729           
=======================================
+ Hits        93745    93754    +9     
+ Misses      47265    47262    -3     
+ Partials     7402     7400    -2     

Flag	Coverage Δ
backend	64.01% <0.00%> (+<0.01%)	⬆️

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[getvictor]

@mna Please review

[mna]

Changes LGTM, I assume you did see the previously-uninstrumented endpoints in APM in your manual QA.