Add smart diff to testutil.GatherAndCompare family of utilities.

[bwplotka]

Currently, it's much better to just use require.Equal from testify to compare outputs of scrape, since it gives nice diff:

0304 12:33:58.224133 4120671 server.go:406] "Adding debug handlers to kubelet server"
    server_test.go:1449: 
        	Error Trace:	server_test.go:1449
        	Error:      	Not equal: 
        	            	...
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -3648,3 +3648,3 @@
        	            	 container_processes{container="",id="/user.slice/user-1000.slice/user@1000.service/session.slice/pulseaudio.service",image="",name="",namespace="",pod=""} 0 1645535816846
        	            	-# HELP container_scrape_error 1 if there was an error while getting container metrics, 0 otherwise.
        	            	+# HELP container_scrape_error 1 if there was an error while getting container metrics, 0 otherwise
        	            	 # TYPE container_scrape_error gauge
        	            	@@ -3939,3 +3939,3 @@
        	            	 container_spec_cpu_period{container="",id="/user.slice/user-1000.slice/user@1000.service/session.slice/pulseaudio.service",image="",name="",namespace="",pod=""} 0
        	            	-# HELP container_spec_cpu_shares CPU share (limit) of the container.
        	            	+# HELP container_spec_cpu_shares CPU share of the container.
        	            	 # TYPE container_spec_cpu_shares gauge
        	Test:       	TestCadvisorMetrics

If I do this with our compare, it is huge unreadable blob without diff ): Just got, want. Let's improve experience here!

[sourikghosh]

@bwplotka you want to change this compare func to use require.Equal

client_golang/prometheus/testutil/testutil.go

Line 200 in 1f81b3e

func compare(got, want []*dto.MetricFamily) error {