Switch from prometheus counters to OTel internally

[AlCutter]

This PR replaces the internal monitoring abstraction, and its prometheus implementation, with the more modern OTel.

This serves a few purposes:
a) Brings this repo into line with the monitoring approach used in our other actively maintained repos.
b) Enables us to add tracing and other OTel supported features.
c) Removes unnecessary complexity stemming from the narrow abstraction (which also serves little purpose as there was only ever one real implementation)

For the moment, the binaries themselves retain their prometheus-compatible HTTP metric export endpoints.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 24.35%. Comparing base (2e3c008) to head (b5fe05f).

Files with missing lines	Patch %	Lines
internal/bastion/bastion.go	0.00%	10 Missing ⚠️
cmd/feedwitness/run_feeders.go	63.63%	6 Missing and 2 partials ⚠️
witness/witness.go	55.55%	4 Missing and 4 partials ⚠️
cmd/feedwitness/main.go	0.00%	5 Missing ⚠️
cmd/omniwitness/monolith.go	0.00%	5 Missing ⚠️
cmd/omniwitness_gcp/main.go	0.00%	5 Missing ⚠️
omniwitness/distribute.go	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #556      +/-   ##
==========================================
+ Coverage   24.12%   24.35%   +0.22%     
==========================================
  Files          30       27       -3     
  Lines        2064     2012      -52     
==========================================
- Hits          498      490       -8     
+ Misses       1469     1417      -52     
- Partials       97      105       +8     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.