Add benchmark for set equality

[dashpole]

Forked from #7175

$ go test -timeout 60s -run=xxxxxMatchNothingxxxxx -test.benchtime=10ms -count 6 -cpu 1 -bench=BenchmarkEquals ./...
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/attribute
cpu: Intel(R) Xeon(R) CPU @ 2.20GHz
BenchmarkEquals/Empty         	 1314884	         9.008 ns/op
BenchmarkEquals/Empty         	 1875915	         8.461 ns/op
BenchmarkEquals/Empty         	 1461613	         7.749 ns/op
BenchmarkEquals/Empty         	 1636912	         9.359 ns/op
BenchmarkEquals/Empty         	 1863039	         6.355 ns/op
BenchmarkEquals/Empty         	 1789053	         6.336 ns/op
BenchmarkEquals/1_string_attribute         	  674168	        16.92 ns/op
BenchmarkEquals/1_string_attribute         	  701983	        16.42 ns/op
BenchmarkEquals/1_string_attribute         	  692001	        16.52 ns/op
BenchmarkEquals/1_string_attribute         	  687970	        16.29 ns/op
BenchmarkEquals/1_string_attribute         	  751766	        16.58 ns/op
BenchmarkEquals/1_string_attribute         	  703534	        16.88 ns/op
BenchmarkEquals/10_string_attributes       	   85400	       137.1 ns/op
BenchmarkEquals/10_string_attributes       	   91045	       136.1 ns/op
BenchmarkEquals/10_string_attributes       	   90973	       150.7 ns/op
BenchmarkEquals/10_string_attributes       	   62877	       177.5 ns/op
BenchmarkEquals/10_string_attributes       	   90780	       194.2 ns/op
BenchmarkEquals/10_string_attributes       	   91058	       144.6 ns/op
BenchmarkEquals/1_int_attribute            	  624625	        18.72 ns/op
BenchmarkEquals/1_int_attribute            	  689478	        16.03 ns/op
BenchmarkEquals/1_int_attribute            	  719173	        15.68 ns/op
BenchmarkEquals/1_int_attribute            	  707005	        16.18 ns/op
BenchmarkEquals/1_int_attribute            	  752048	        15.94 ns/op
BenchmarkEquals/1_int_attribute            	  752034	        16.23 ns/op
BenchmarkEquals/10_int_attributes          	   90302	       132.5 ns/op
BenchmarkEquals/10_int_attributes          	   89929	       131.9 ns/op
BenchmarkEquals/10_int_attributes          	   86578	       135.2 ns/op
BenchmarkEquals/10_int_attributes          	   90482	       133.1 ns/op
BenchmarkEquals/10_int_attributes          	   90255	       132.0 ns/op
BenchmarkEquals/10_int_attributes          	   87615	       134.6 ns/op
PASS
ok  	go.opentelemetry.io/otel/attribute	0.578s
PASS
ok  	go.opentelemetry.io/otel/attribute/internal	0.017s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.8%. Comparing base (5358fd7) to head (6313357).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7262     +/-   ##
=======================================
- Coverage   82.8%   82.8%   -0.1%     
=======================================
  Files        265     265             
  Lines      24896   24896             
=======================================
- Hits       20631   20628      -3     
- Misses      3885    3888      +3     
  Partials     380     380             

see 2 files with indirect coverage changes

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