make probe_http_requests_failed_total a counter instead of a rate

`probe_http_requests_failed` is emitted as a rate now, as the original
metric is

Author: nadiamoe

integration/integration_test.go

@@ -83,7 +83,8 @@ func TestSMK6(t *testing.T) {
 			"probe_http_error_code",
 			"probe_http_got_expected_response",
 			"probe_http_info",
-			"probe_http_requests_failed_total",
+			"probe_http_requests_failed",       // Original rate.
+			"probe_http_requests_failed_total", // Computed counter.
 			"probe_http_requests_total",
 			"probe_http_ssl",
 			"probe_http_status_code",
@@ -107,7 +108,7 @@ func TestSMK6(t *testing.T) {
 
 		unwantedMetrics := []string{
 			"probe_checks",
-			"probe_http_reqs", "probe_http_req_failed",
+			"probe_http_reqs", "probe_http_req_failed", // Renamed s/req/requests.
 			"probe_data_sent", "probe_data_received",
 			"probe_http_req_duration", "probe_iteration_duration",
 			"probe_http_req_blocked", "probe_http_req_connecting", "probe_http_req_receiving", "probe_http_req_sending", "probe_http_req_tls_handshaking", "probe_http_req_waiting",
@@ -282,9 +283,28 @@ func TestSMK6(t *testing.T) {
 				assertValue:  equals(0),
 			},
 			{
-				name:        "Total requests for each url",
-				metricName:  "probe_http_requests_total",
-				assertValue: equals(1),
+				name:         "Total requests for a URL accessed once",
+				metricName:   "probe_http_requests_total",
+				metricLabels: map[string]string{"url": "https://test-api.k6.io/public/crocodiles/"},
+				assertValue:  equals(1),
+			},
+			{
+				name:         "Total requests for a URL accessed twice",
+				metricName:   "probe_http_requests_total",
+				metricLabels: map[string]string{"url": "https://test-api.k6.io/public/crocodiles4/"},
+				assertValue:  equals(2),
+			},
+			{
+				name:         "HTTP requests failed rate",
+				metricName:   "probe_http_requests_failed",
+				metricLabels: map[string]string{"url": "https://test-api.k6.io/public/crocodiles4/"},
+				assertValue:  equals(1),
+			},
+			{
+				name:         "HTTP requests failed ttoal",
+				metricName:   "probe_http_requests_failed_total",
+				metricLabels: map[string]string{"url": "https://test-api.k6.io/public/crocodiles4/"},
+				assertValue:  equals(2),
 			},
 			{
 				name:         "HTTP version",

integration/test-script.js

@@ -41,5 +41,6 @@ export default function () {
   http.get(`https://${testHost}/public/crocodiles2/`); // 404
   http.get(`https://${testHost}/public/crocodiles3/`); // 404
   http.get(`https://${testHost}/public/crocodiles4/`); // 404
+  http.get(`https://${testHost}/public/crocodiles4/`); // Second 404, to assert differences between failure rate and counter.
   http.get(`http://fail.internal/public/crocodiles4/`); // failed
 }

output.go

@@ -253,12 +253,27 @@ func (ms *metricStore) DeriveMetrics() {
 				log.Tracef("Created %q from %q", httpVersionTS.name, ts.name)
 			}()
 
-		// Rename to http_requests_failed_total
+		// http_req_failed is a rate, and traditionally we have reported http_requests_failed_total (as a counter)
+		// Here we derive the total from the rate.
 		case "http_req_failed":
-			newTS := ts
-			newTS.name = "http_requests_failed_total"
-			ms.store[newTS] = v
-			log.Tracef("Created %q from %q", newTS.name, ts.name)
+			// Derive the counter
+			func() {
+				failedTotal := ts
+				failedTotal.name = "http_requests_failed_total"
+				ms.store[failedTotal] = value{
+					// Number of failures is the (computed) avg failure rate times the number of samples.
+					value:       math.Round(float64(v.seenSamples) * v.value),
+					seenSamples: 1,
+				}
+				log.Tracef("Created %q from %q", failedTotal.name, ts.name)
+			}()
+			// Also rename it to s/req/requests.
+			func() {
+				newTS := ts
+				newTS.name = "http_requests_failed"
+				ms.store[newTS] = v
+				log.Tracef("Created %q from %q", newTS.name, ts.name)
+			}()
 
 		// Add _bytes suffix to data_sent and data_received.
 		case "data_sent", "data_received":
@@ -426,7 +441,7 @@ func (ms *metricStore) RemoveMetrics() {
 		"iteration_duration": true,
 		// Squashed into a single metric with a phase label.
 		"http_req_blocked": true, "http_req_connecting": true, "http_req_receiving": true, "http_req_sending": true, "http_req_tls_handshaking": true, "http_req_waiting": true,
-		// Renamed to http_requests(_failed)_total.
+		// Renamed s/reqs/requests.
 		"http_reqs":       true,
 		"http_req_failed": true,
 		// Replaced by check_rate and checks_total