Metrics: Add push client to push metrics to endpoint

Creates a push client that will be used to push metrics to
Prometheus Aggregation Gateway. The aggregation gateway expects
a specific query path to push the metrics to and hence a client is
created which will format the metrics as per the aggregation gateway's
requirement.

Also added extra helper functions that will be used by the metrics API
to push to the gateway.

go.mod

@@ -80,7 +80,8 @@ require (
 	github.com/pierrec/lz4 v2.3.0+incompatible // indirect
 	github.com/pkg/errors v0.9.1
 	github.com/pkg/sftp v1.10.1
-	github.com/prometheus/client_golang v1.2.1 // indirect
+	github.com/prometheus/client_golang v1.2.1
+	github.com/prometheus/common v0.7.0
 	github.com/prometheus/procfs v0.0.8 // indirect
 	github.com/satori/uuid v1.2.0 // indirect
 	github.com/shurcooL/httpfs v0.0.0-20190707220628-8d4bc4ba7749 // indirect

pkg/metrics/pushclient/pushclient.go

@@ -0,0 +1,38 @@
+package pushclient
+
+import (
+	"net/http"
+
+	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/push"
+	"github.com/prometheus/common/expfmt"
+)
+
+// gatewayURL is the default url that will be used as the endpoint to push to.
+// To push to a centralized prometheus aggregation gateway, change this url.
+var gatewayURL = "http://localhost:9092"
+
+// pushAllToAggregationGateway is a helper that takes care of the actual code that pushes to the prometheus
+// aggregation gateway. It takes a list of collectors and pushes all of them to the desired url.
+func pushAllToAggregationGateway(url string, client push.HTTPDoer, collectors ...prometheus.Collector) error {
+	allJobsName := "installer_metrics"
+	pushClient := push.New(url, allJobsName).Client(client).Format(expfmt.FmtText)
+
+	for _, value := range collectors {
+		pushClient.Collector(value)
+	}
+
+	err := pushClient.Push()
+	if err != nil {
+		return errors.Wrap(err, "failed to push metrics")
+	}
+	return nil
+}
+
+// Push calls the pushAllToAggregationGateway for the all metrics.
+// This function acts as a re-router which currently pushes to the aggregation gateway.
+// Any change in gateway logic can be written here to route the metrics.
+func Push(collectors ...prometheus.Collector) error {
+	return pushAllToAggregationGateway(gatewayURL, &http.Client{}, collectors...)
+}

pkg/metrics/pushclient/pushclient_test.go

@@ -0,0 +1,94 @@
+package pushclient
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPushMetricsToGateway(t *testing.T) {
+	promCollector := prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Name:        "test",
+			Help:        "test metrics",
+			ConstLabels: map[string]string{"test1": "test1"},
+		},
+	)
+
+	promCollector.Add(10)
+
+	expectedOutput := `# HELP test test metrics
+# TYPE test counter
+test{test1="test1"} 10
+`
+
+	testServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		buf := new(strings.Builder)
+		_, err := io.Copy(buf, req.Body)
+		assert.NoError(t, err)
+		assert.EqualValues(t, expectedOutput, buf.String())
+		fmt.Println("all done")
+	}))
+	defer testServer.Close()
+	pushAllToAggregationGateway(testServer.URL, &aggregationClient{&http.Client{}}, promCollector)
+
+}
+
+func TestPushMetricsToAllGateway(t *testing.T) {
+	counter := prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Name:        "test_counter",
+			Help:        "test metrics",
+			ConstLabels: map[string]string{"test1": "test1"},
+		},
+	)
+	counter.Add(10)
+
+	histogram := prometheus.NewHistogram(
+		prometheus.HistogramOpts{
+			Name:        "test_histogram",
+			Help:        "test metrics",
+			ConstLabels: map[string]string{"test1": "test1"},
+		},
+	)
+	histogram.Observe(10)
+
+	promCollector := []prometheus.Collector{counter, histogram}
+
+	expectedOutput := `# HELP test_counter test metrics
+# TYPE test_counter counter
+test_counter{test1="test1"} 10
+# HELP test_histogram test metrics
+# TYPE test_histogram histogram
+test_histogram_bucket{test1="test1",le="0.005"} 0
+test_histogram_bucket{test1="test1",le="0.01"} 0
+test_histogram_bucket{test1="test1",le="0.025"} 0
+test_histogram_bucket{test1="test1",le="0.05"} 0
+test_histogram_bucket{test1="test1",le="0.1"} 0
+test_histogram_bucket{test1="test1",le="0.25"} 0
+test_histogram_bucket{test1="test1",le="0.5"} 0
+test_histogram_bucket{test1="test1",le="1"} 0
+test_histogram_bucket{test1="test1",le="2.5"} 0
+test_histogram_bucket{test1="test1",le="5"} 0
+test_histogram_bucket{test1="test1",le="10"} 1
+test_histogram_bucket{test1="test1",le="+Inf"} 1
+test_histogram_sum{test1="test1"} 10
+test_histogram_count{test1="test1"} 1
+`
+
+	testServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		buf := new(strings.Builder)
+		_, err := io.Copy(buf, req.Body)
+		assert.NoError(t, err)
+		assert.EqualValues(t, expectedOutput, buf.String())
+		fmt.Println("all done")
+	}))
+	defer testServer.Close()
+	pushAllToAggregationGateway(testServer.URL, &aggregationClient{&http.Client{}}, promCollector...)
+}