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 client can be used for
any type of Gateway and it takes in the URL and the HTTP Client
that would have all the necessary information(authentication)
required to push to the gateway.

Configured to take in Prometheus Objects for pushing to gateway.
https://github.com/prometheus/client_golang/tree/master/prometheus

go.mod

@@ -64,6 +64,8 @@ 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 v1.5.1
+	github.com/prometheus/common v0.9.1
 	github.com/satori/uuid v1.2.0 // indirect
 	github.com/shurcooL/vfsgen v0.0.0-20181202132449-6a9ea43bcacd
 	github.com/sirupsen/logrus v1.5.0

pkg/metrics/pushclient/pushclient.go

@@ -0,0 +1,42 @@
+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"
+)
+
+// PushClient is used to send Prometheus metrics objects to any Prometheus
+// Push Gateway. It stores the URL and the client that will be used to push
+// to the desired gateway.
+type PushClient struct {
+	URL    string
+	Client *http.Client
+}
+
+// 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 (p *PushClient) pushAllToAggregationGateway(collectors ...prometheus.Collector) error {
+	allJobsName := "installer_metrics"
+	pushClient := push.New(p.URL, allJobsName).Client(p.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 (p *PushClient) Push(collectors ...prometheus.Collector) error {
+	return p.pushAllToAggregationGateway(collectors...)
+}

pkg/metrics/pushclient/pushclient_test.go

@@ -0,0 +1,97 @@
+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()
+
+	pushClient := PushClient{URL: testServer.URL, Client: &http.Client{}}
+	pushClient.Push(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()
+	pushClient := PushClient{URL: testServer.URL, Client: &http.Client{}}
+	pushClient.Push(promCollector...)
+}