Backend: Expose prometheus-style metrics endpoint

Expose a new API which can be scraped by prometheus to gather useful
metrics from an instance. The new endpoint exposes photoprism build
version information, golang version, edition and various count metrics.

internal/api/api_test.go

@@ -15,6 +15,19 @@ import (
 	"github.com/photoprism/photoprism/internal/get"
 )
 
+type CloseableResponseRecorder struct {
+	*httptest.ResponseRecorder
+	closeCh chan bool
+}
+
+func (r *CloseableResponseRecorder) CloseNotify() <-chan bool {
+	return r.closeCh
+}
+
+func (r *CloseableResponseRecorder) closeClient() {
+	r.closeCh <- true
+}
+
 func TestMain(m *testing.M) {
 	log = logrus.StandardLogger()
 	log.SetLevel(logrus.TraceLevel)
@@ -60,3 +73,13 @@ func PerformRequestWithBody(r http.Handler, method, path, body string) *httptest
 
 	return w
 }
+
+// Executes an API request with a stream response.
+func PerformRequestWithStream(r http.Handler, method, path string) *CloseableResponseRecorder {
+	req, _ := http.NewRequest(method, path, nil)
+	w := &CloseableResponseRecorder{httptest.NewRecorder(), make(chan bool, 1)}
+
+	r.ServeHTTP(w, req)
+
+	return w
+}