chore(prometheus): restore crate to workspace

[cijothomas]

Changes

Un-deprecates opentelemetry-prometheus by removing stale unsupported/discontinued notices and restoring the crate to the workspace. Sets the crate MSRV to 1.81.0 to match the prometheus dependency requirement.

Fixes #3288

Validation

• cargo metadata --no-deps --format-version 1
• cargo +1.81.0 check -p opentelemetry-prometheus --all-targets --all-features
• cargo +nightly shear --expand
• cargo clippy -p opentelemetry-prometheus --all-targets --all-features -- -Dwarnings
• cargo test -p opentelemetry-prometheus --all-features --lib

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.8%. Comparing base (cc87dd9) to head (74a081b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3500     +/-   ##
=======================================
- Coverage   84.1%   82.8%   -1.3%     
=======================================
  Files        126     130      +4     
  Lines      26720   27276    +556     
=======================================
+ Hits       22490   22609    +119     
- Misses      4230    4667    +437     

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