otellogr: Add benchmarks

[scorpionknifes]

Add benchmarks (similar to otelslog)
Part of #5192

Benchmark results:

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/contrib/bridges/otellogr
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
BenchmarkLogSink/Info-16                 6114177               186.4 ns/op           160 B/op          1 allocs/op
BenchmarkLogSink/Error-16                6649935               245.2 ns/op           160 B/op          1 allocs/op
BenchmarkLogSink/WithValues-16           3949730               295.6 ns/op           448 B/op          3 allocs/op
BenchmarkLogSink/WithName-16             6813362               172.0 ns/op           133 B/op          2 allocs/op
BenchmarkLogSink/WithName.WithValues-16                  2420191               606.4 ns/op           581 B/op          5 allocs/op
BenchmarkLogSink/(WithName.WithValues).Info-16          12601381                97.32 ns/op            0 B/op          0 allocs/op
PASS
ok      go.opentelemetry.io/contrib/bridges/otellogr    24.125s

[dmathieu]

Could you add the result of those benchmarks to the PR description ?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.9%. Comparing base (0da64d1) to head (719992b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6364   +/-   ##
=====================================
  Coverage   66.9%   66.9%           
=====================================
  Files        193     193           
  Lines      15652   15652           
=====================================
+ Hits       10480   10483    +3     
+ Misses      4881    4879    -2     
+ Partials     291     290    -1     

see 1 file with indirect coverage changes