internal/frontend: add metrics for playground share requests

A metric is added for everytime a request is sent to play.golang.org.
The request status code will be used and is useful to have better
visibility on how many users click the new playground "play" button.

For golang/go#36865

Change-Id: I243efe5b3d8724fa6bdd60d332e2932925ffe094
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/245898
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: Miguel Acero

cmd/frontend/main.go

@@ -131,6 +131,7 @@ func main() {
 		postgres.SearchResponseCount,
 		frontend.FetchLatencyDistribution,
 		frontend.FetchResponseCount,
+		frontend.PlaygroundShareRequestCount,
 		middleware.CacheResultCount,
 		middleware.CacheErrorCount,
 		middleware.QuotaResultCount,

internal/frontend/playground.go

@@ -7,13 +7,34 @@ package frontend
 import (
 	"io"
 	"net/http"
+	"strconv"
 
+	"go.opencensus.io/stats"
+	"go.opencensus.io/stats/view"
+	"go.opencensus.io/tag"
 	"golang.org/x/pkgsite/internal/log"
 )
 
 // playgroundURL is the playground endpoint used for share links.
 const playgroundURL = "https://play.golang.org"
 
+var (
+	keyPlaygroundShareStatus = tag.MustNewKey("playground.share.status")
+	playgroundShareStatus    = stats.Int64(
+		"go-discovery/playground_share_count",
+		"The status of a request to play.golang.org/share",
+		stats.UnitDimensionless,
+	)
+
+	PlaygroundShareRequestCount = &view.View{
+		Name:        "go-discovery/playground/share_count",
+		Measure:     playgroundShareStatus,
+		Aggregation: view.Count(),
+		Description: "Playground share request count",
+		TagKeys:     []tag.Key{keyPlaygroundShareStatus},
+	}
+)
+
 // handlePlay handles requests that mirror play.golang.org/share.
 func (s *Server) handlePlay(w http.ResponseWriter, r *http.Request) {
 	makeFetchPlayRequest(w, r, playgroundURL)
@@ -43,6 +64,10 @@ func makeFetchPlayRequest(w http.ResponseWriter, r *http.Request, pgURL string)
 		httpErrorStatus(w, http.StatusInternalServerError)
 		return
 	}
+	stats.RecordWithTags(r.Context(),
+		[]tag.Mutator{tag.Upsert(keyPlaygroundShareStatus, strconv.Itoa(resp.StatusCode))},
+		playgroundShareStatus.M(int64(resp.StatusCode)),
+	)
 	copyHeader := func(k string) {
 		if v := resp.Header.Get(k); v != "" {
 			w.Header().Set(k, v)