(api): Update metrics endpoint to return multiple metrics

metrics_handler.go

@@ -1,6 +1,7 @@
 package asynqmon
 
 import (
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
@@ -11,7 +12,7 @@ import (
 )
 
 type getMetricsResponse struct {
-	// TODO
+	QueueSize *json.RawMessage `json:"queue_size"`
 }
 
 type metricsFetchOptions struct {
@@ -32,16 +33,31 @@ func newGetMetricsHandlerFunc(client *http.Client, prometheusAddr string) http.H
 			http.Error(w, fmt.Sprintf("invalid query parameter: %v", err), http.StatusBadRequest)
 			return
 		}
-		url := buildPrometheusURL(prometheusAddr, "asynq_queue_size", opts)
+		// List of quries to send to prometheus server.
-		fmt.Printf("DEBUG: url: %s\n", url)
+		queries := []string{
-		resp, err := client.Get(url)
+			"asynq_queue_size",
+		}
+		resp := getMetricsResponse{}
+		for _, q := range queries {
+			url := buildPrometheusURL(prometheusAddr, q, opts)
+			fmt.Printf("DEBUG: url: %s\n", url)
+			msg, err := fetchPrometheusMetrics(client, url)
+			if err != nil {
+				http.Error(w, fmt.Sprintf("failed to fetch %q: %v", url, err), http.StatusInternalServerError)
+				return
+			}
+			switch q {
+			case "asynq_queue_size":
+				resp.QueueSize = msg
+			}
+		}
+		bytes, err := json.Marshal(resp)
 		if err != nil {
-			http.Error(w, fmt.Sprintf("request failed: GET %q: %v", url, err), http.StatusInternalServerError)
+			http.Error(w, fmt.Sprintf("failed to marshal response into JSON: %v", err), http.StatusInternalServerError)
 			return
 		}
-		defer resp.Body.Close()
+		if _, err := w.Write(bytes); err != nil {
-		if _, err := io.Copy(w, resp.Body); err != nil {
+			http.Error(w, fmt.Sprintf("failed to write to response: %v", err), http.StatusInternalServerError)
-			http.Error(w, fmt.Sprintf("failed to copy: %v", err), http.StatusInternalServerError)
 			return
 		}
 	}
@@ -86,6 +102,20 @@ func buildPrometheusURL(baseAddr, promQL string, opts *metricsFetchOptions) stri
 	return b.String()
 }
 
+func fetchPrometheusMetrics(client *http.Client, url string) (*json.RawMessage, error) {
+	resp, err := client.Get(url)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	bytes, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	msg := json.RawMessage(bytes)
+	return &msg, err
+}
+
 // Returns step to use given the fetch options.
 // In general, the longer the duration, longer the each step.
 func step(opts *metricsFetchOptions) time.Duration {