Deadlock when using invalid metric name with prometheus reporter

[aud10slave]

If prometheus reporter is used and and an invalid metrics name is supplied then the scope.Close goes into a deadlock.

Example code:

var reporter tally.CachedStatsReporter
promReporter := tallyprom.NewReporter(tallyprom.Options{})
reporter = tallymulti.NewMultiCachedReporter(promReporter)
scope, closer := tally.NewRootScope(
	tally.ScopeOptions{
		Prefix:         "test",
		Tags:           map[string]string{},
		CachedReporter: reporter,
	},
	1 * time.Second)
defer closer.Close()

// use the scope 
g := scope.Gauge("invalid-name")
fmt.Println("gauge created")

The Println never gets called. Looking at the goroutine dump here's what happens

1. scope.Gauge acquires a write lock on the gauge mutex

2. prometheus reporter panics for an invalid gauge name

3. This causes the defers on the stack to be called including closer.Close()

4. scope.Close() tries to acquire the lock again via reportRegistryWithLock

If defer closer.Close() is moved after calling scope.Gauge then the error is reported.

One possible solution is to call gm.Unlock() using defer as well in scope.Gauge