Add benchmark for set equality (#7262)

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
```

---------

Co-authored-by: Flc゛ <[email protected]>
Co-authored-by: Robert Pająk <[email protected]>

Author: 3 people

attribute/benchmark_test.go

@@ -272,6 +272,59 @@ func BenchmarkStringSlice(b *testing.B) {
 	b.Run("Emit", benchmarkEmit(kv))
 }
 
+func BenchmarkSetEquals(b *testing.B) {
+	b.Run("Empty", func(b *testing.B) {
+		benchmarkSetEquals(b, attribute.EmptySet())
+	})
+	b.Run("1 string attribute", func(b *testing.B) {
+		set := attribute.NewSet(attribute.String("string", "42"))
+		benchmarkSetEquals(b, &set)
+	})
+	b.Run("10 string attributes", func(b *testing.B) {
+		set := attribute.NewSet(
+			attribute.String("a", "42"),
+			attribute.String("b", "42"),
+			attribute.String("c", "42"),
+			attribute.String("d", "42"),
+			attribute.String("e", "42"),
+			attribute.String("f", "42"),
+			attribute.String("g", "42"),
+			attribute.String("h", "42"),
+			attribute.String("i", "42"),
+			attribute.String("j", "42"),
+		)
+		benchmarkSetEquals(b, &set)
+	})
+	b.Run("1 int attribute", func(b *testing.B) {
+		set := attribute.NewSet(attribute.Int("string", 42))
+		benchmarkSetEquals(b, &set)
+	})
+	b.Run("10 int attributes", func(b *testing.B) {
+		set := attribute.NewSet(
+			attribute.Int("a", 42),
+			attribute.Int("b", 42),
+			attribute.Int("c", 42),
+			attribute.Int("d", 42),
+			attribute.Int("e", 42),
+			attribute.Int("f", 42),
+			attribute.Int("g", 42),
+			attribute.Int("h", 42),
+			attribute.Int("i", 42),
+			attribute.Int("j", 42),
+		)
+		benchmarkSetEquals(b, &set)
+	})
+}
+
+func benchmarkSetEquals(b *testing.B, set *attribute.Set) {
+	b.ResetTimer()
+	for range b.N {
+		if !set.Equals(set) {
+			b.Fatal("not equal")
+		}
+	}
+}
+
 // BenchmarkEquivalentMapAccess measures how expensive it is to use
 // Equivalent() as a map key. This is on the hot path for making synchronous
 // measurements on the metrics API/SDK. It will likely be on the hot path for