Metrics registered on wrong prometheus registry (#2246)

* - UT on metrics verifying that all plugins of all blocs have their metrics collectors declared

* - fix error msg

* - redirect Registry of metric to the one that handle the listener
- allow duplicate of metrics collector on the same Registry (case of same plugin in 2 blocs listening metrics on the same address)

* - fix change of signature

* - ensure cleaning metrics before starting the test (metrics collectors are global vars .. and re-used by several tests)

* - I think I fixed this test. Ensure correct mn of hits and clean metrics before test.

* - fix typo in error msg - proposed at review

* - fix typo in comment

* - remove ResetMetrics functions
- change a way to test the numeric metrics : get the diff between begin and end of test

* - oops. removing debug logs

plugin/pkg/uniq/uniq_test.go

@@ -4,7 +4,7 @@ import "testing"
 
 func TestForEach(t *testing.T) {
 	u, i := New(), 0
-	u.Set("test", func() error { i++; return nil })
+	u.Set("test", func() error { i++; return nil }, nil)
 
 	u.ForEach()
 	if i != 1 {