prometheus: read from file (#1453)

modules/prometheus/metadata.yaml

@@ -186,6 +186,13 @@ modules:
                 jobs:
                   - name: local
                     url: http://127.0.0.1:9090/metrics
+            - name: Read metrics from a file
+              description: An example configuration to read metrics from a file.
+              config: |
+                # use "file://" scheme
+                jobs:
+                  - name: myapp
+                    url: file:///opt/metrics/myapp/metrics.txt
             - name: HTTP authentication
               description: |
                 > **Note**: Change the port of the monitored application on which it provides metrics.

pkg/prometheus/client.go

@@ -9,6 +9,9 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
+	"os"
+	"path/filepath"
 
 	"github.com/netdata/go.d.plugin/pkg/prometheus/selector"
 	"github.com/netdata/go.d.plugin/pkg/web"
@@ -24,8 +27,9 @@ type (
 	}
 
 	prometheus struct {
-		client  *http.Client
-		request web.Request
+		client   *http.Client
+		request  web.Request
+		filepath string
 
 		sr selector.Selector
 
@@ -53,13 +57,19 @@ func New(client *http.Client, request web.Request) Prometheus {
 
 // NewWithSelector creates a Prometheus instance with the selector.
 func NewWithSelector(client *http.Client, request web.Request, sr selector.Selector) Prometheus {
-	return &prometheus{
+	p := &prometheus{
 		client:  client,
 		request: request,
 		sr:      sr,
 		buf:     bytes.NewBuffer(make([]byte, 0, 16000)),
 		parser:  promTextParser{sr: sr},
 	}
+
+	if v, err := url.Parse(request.URL); err == nil && v.Scheme == "file" {
+		p.filepath = filepath.Join(v.Host, v.Path)
+	}
+
+	return p
 }
 
 func (p *prometheus) HTTPClient() *http.Client {
@@ -88,6 +98,19 @@ func (p *prometheus) Scrape() (MetricFamilies, error) {
 }
 
 func (p *prometheus) fetch(w io.Writer) error {
+	// TODO: should be a separate text file prom client
+	if p.filepath != "" {
+		f, err := os.Open(p.filepath)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+
+		_, err = io.Copy(w, f)
+
+		return err
+	}
+
 	req, err := web.NewHTTPRequest(p.request)
 	if err != nil {
 		return err

pkg/prometheus/client_test.go

@@ -111,6 +111,17 @@ func TestPrometheusGzip(t *testing.T) {
 	}
 }
 
+func TestPrometheusReadFromFile(t *testing.T) {
+	req := web.Request{URL: "file://testdata/testdata.txt"}
+	prom := NewWithSelector(http.DefaultClient, req, nil)
+
+	for i := 0; i < 2; i++ {
+		res, err := prom.ScrapeSeries()
+		assert.NoError(t, err)
+		verifyTestData(t, res)
+	}
+}
+
 func verifyTestData(t *testing.T, ms Series) {
 	assert.Equal(t, 410, len(ms))
 	assert.Equal(t, "go_gc_duration_seconds", ms[0].Labels.Get("__name__"))