feat(push): add format builder option

Allow users to specify which format the pusher should push in.

Signed-off-by: Frank Spitulski <fspituls@ucsd.edu>

prometheus/push/push.go

@@ -64,6 +64,8 @@ type Pusher struct {
 	client             *http.Client
 	useBasicAuth       bool
 	username, password string
+
+	expfmt expfmt.Format
 }
 
 // New creates a new Pusher to push to the provided URL with the provided job
@@ -96,6 +98,7 @@ func New(url, job string) *Pusher {
 		gatherers:  prometheus.Gatherers{reg},
 		registerer: reg,
 		client:     &http.Client{},
+		expfmt:     expfmt.FmtProtoDelim,
 	}
 }
 
@@ -182,6 +185,14 @@ func (p *Pusher) BasicAuth(username, password string) *Pusher {
 	return p
 }
 
+// Format configures the Pusher to use an encoding format given by the
+// provided expfmt.Format. For convenience, this method returns a
+// pointer to the Pusher itself.
+func (p *Pusher) Format(format expfmt.Format) *Pusher {
+	p.expfmt = format
+	return p
+}
+
 func (p *Pusher) push(method string) error {
 	if p.error != nil {
 		return p.error
@@ -197,7 +208,7 @@ func (p *Pusher) push(method string) error {
 		return err
 	}
 	buf := &bytes.Buffer{}
-	enc := expfmt.NewEncoder(buf, expfmt.FmtProtoDelim)
+	enc := expfmt.NewEncoder(buf, p.expfmt)
 	// Check for pre-existing grouping labels:
 	for _, mf := range mfs {
 		for _, m := range mf.GetMetric() {