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

example/prometheus/main.go

@@ -0,0 +1,105 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"time"
+
+	"go.opentelemetry.io/otel/api/global"
+	"go.opentelemetry.io/otel/api/key"
+	"go.opentelemetry.io/otel/api/metric"
+	"go.opentelemetry.io/otel/exporter/metric/prometheus"
+	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/metric/batcher/defaultkeys"
+	"go.opentelemetry.io/otel/sdk/metric/controller/push"
+	"go.opentelemetry.io/otel/sdk/metric/selector/simple"
+)
+
+var (
+	fooKey    = key.New("ex.com/foo")
+	barKey    = key.New("ex.com/bar")
+	lemonsKey = key.New("ex.com/lemons")
+)
+
+func initMeter() *push.Controller {
+	selector := simple.NewWithExactMeasure()
+	exporter, err := prometheus.NewExporter(prometheus.Options{
+		DefaultHistogramBuckets: []float64{0., 10., 15., 20.},
+	})
+
+	if err != nil {
+		log.Panicf("failed to initialize metric stdout exporter %v", err)
+	}
+	batcher := defaultkeys.New(selector, sdkmetric.NewDefaultLabelEncoder(), false)
+	pusher := push.New(batcher, exporter, time.Second)
+	pusher.Start()
+
+	go func() {
+		_ = http.ListenAndServe(":2222", exporter)
+	}()
+
+	global.SetMeterProvider(pusher)
+	return pusher
+}
+
+func main() {
+	defer initMeter().Stop()
+
+	meter := global.MeterProvider().Meter("ex.com/basic")
+
+	oneMetric := meter.NewFloat64Gauge("ex.com.one",
+		metric.WithKeys(fooKey, barKey, lemonsKey),
+		metric.WithDescription("A gauge set to 1.0"),
+	)
+
+	measureTwo := meter.NewFloat64Measure("ex.com.two", metric.WithKeys(key.New("A")))
+	measureThree := meter.NewFloat64Counter("ex.com.three")
+
+	commonLabels := meter.Labels(lemonsKey.Int(10), key.String("A", "1"), key.String("B", "2"), key.String("C", "3"))
+	notSoCommonLabels := meter.Labels(lemonsKey.Int(13))
+
+	ctx := context.Background()
+
+	meter.RecordBatch(
+		ctx,
+		commonLabels,
+		oneMetric.Measurement(1.0),
+		measureTwo.Measurement(2.0),
+		measureThree.Measurement(12.0),
+	)
+
+	meter.RecordBatch(
+		ctx,
+		notSoCommonLabels,
+		oneMetric.Measurement(1.0),
+		measureTwo.Measurement(2.0),
+		measureThree.Measurement(22.0),
+	)
+
+	time.Sleep(5 * time.Second)
+
+	meter.RecordBatch(
+		ctx,
+		commonLabels,
+		oneMetric.Measurement(13.0),
+		measureTwo.Measurement(12.0),
+		measureThree.Measurement(13.0),
+	)
+
+	time.Sleep(100 * time.Second)
+}