Prometheus exporter (#334)

* Prom exporter structure

* update prometheus exporter with master and add example.

* remove distributedcontext from prometheus example

* docs and interface checker

* make precommit

* make precommit & remove "OnRegisterError"

* coerce values to float

* return register errors and maybe fix precommit?

* add option to specify a prometheus.Registry

* make exporter implement http.Handler interface

* fix map keys bugs

* remove unused const

* fix modules dependencies.

* add support for histogram

* get metrics with labels values only instead of a labels map

* make exporter implements label encoder interface

* encode labels if the encoder is different.

* split metrics on several files and encapsulate them in structs

* make pre commit

* unexport 'sanitize'

* remove 'AllValues' in favor of 'Points' and change to 'NewDefaultLabelEncoder'

* add prometheus tests

* remove newlines on struct declaration

* formatting

* rewording

* imports

* add todo on labelValues

* blame myself for todo (:

* add todos on sanitize

* add support for summaries. custom remove label encoder.

* imports

* imports

* update with upstream

sdk/metric/aggregator/array/array_test.go

@@ -305,6 +305,13 @@ func TestArrayFloat64(t *testing.T) {
 			qx, err := agg.Quantile(0.5)
 			require.Nil(t, err)
 			require.Equal(t, all.Median(), qx, "Same median")
+
+			po, err := agg.Points()
+			require.Nil(t, err)
+			require.Equal(t, all.Len(), len(po), "Points() must have same length of updates")
+			for i := 0; i < len(po); i++ {
+				require.Equal(t, all.Points()[i], po[i], "Wrong point at position %d", i)
+			}
 		})
 	}
 }