Add ObserverEffect and Nil* metrics.

[rcrowley]

See the comment on ObserverEffect in metrics.go for more.

Note that this changes the signature of New* for all metrics to return the interface type. I remember a discussion on the mailing list long ago that convinced me to change the return value of those functions to a concrete type but I can't recall why and this is a good reason to change it back.

[mihasya]

Would love to see the mailing list discussion that made you go with a regular struct.

This looks good, other than the fact that you're breaking master Of course, I encourage this as each time this happens, we move closer to a point where the community starts to converge on a solution for versioning.

[rcrowley]

https://groups.google.com/forum/#!msg/golang-nuts/t90AIEpb-ao/1Q0sEsw_efIJ

[wadey]

I like this and the patch looks good. I'm not sure I like the name ObserverEffect though, seems too complicated and not obvious. Why not something simple like DisableMetrics or UseNilMetrics.

[rcrowley]

Alright, ETOOCLEVER, I'll change it.

[wadey]

This is cool trick, but unnecessary here since NewCounter will enforce the same thing.

[mihasya]

I actually was gonna say something about ObserverEffect but then I
remembered getFletcherFromDayCare and decided I had no platform whence to
speak.

On Fri, Sep 13, 2013 at 11:51 AM, Wade Simmons notifications@github.comwrote:

In counter.go:

@@ -13,6 +13,32 @@ type Counter interface {
Inc(int64)
}

+// Create a new Counter.
+func NewCounter() Counter {

• if UseNilMetrics {

•   return NilCounter{}
  

• }
• return &StandardCounter{0}
  +}

+// No-op Counter.
+type NilCounter struct{}
+
+// Force the compiler to check that NilCounter implements Counter.
+var _ Counter = NilCounter{}

This is cool trick, but unnecessary here since NewCounter will enforce the
same thing.

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