Inhibition Rules: Add benchmarks for Mutes

[grobinson-grafana]

This commit adds benchmarks for Mutes in the inhibit package. Here is an example of the benchmarks being run:

go test ./... -bench=.
goos: darwin
goarch: arm64
pkg: github.com/prometheus/alertmanager/inhibit
BenchmarkMutes/1_inhibition_rule,_1_inhibiting_alert-8         	 1498118	       778.2 ns/op
BenchmarkMutes/10_inhibition_rules,_1_inhibiting_alert-8       	 1528195	       784.1 ns/op
BenchmarkMutes/100_inhibition_rules,_1_inhibiting_alert-8      	 1364626	       797.3 ns/op
BenchmarkMutes/1000_inhibition_rules,_1_inhibiting_alert-8     	 1331232	       848.7 ns/op
BenchmarkMutes/10000_inhibition_rules,_1_inhibiting_alert-8    	 1314247	       841.0 ns/op
BenchmarkMutes/1_inhibition_rule,_10_inhibiting_alerts-8       	 1379548	       870.3 ns/op
BenchmarkMutes/1_inhibition_rule,_100_inhibiting_alerts-8      	  488551	      2188 ns/op
BenchmarkMutes/1_inhibition_rule,_1000_inhibiting_alerts-8     	   98607	     12039 ns/op
BenchmarkMutes/1_inhibition_rule,_10000_inhibiting_alerts-8    	   10754	    104250 ns/op
BenchmarkMutes/100_inhibition_rules,_1000_inhibiting_alerts-8  	  103459	     11382 ns/op
PASS
ok  	github.com/prometheus/alertmanager/inhibit	44.303s

[gotjosh]

LGTM