[340] fix bug in `testApp.metricsResource`

* it omitted a `gauges` field, causing integration tests that called
  `metrics.setGauge` to incorrectly (and confusingly!) fail

test/support/testApp.js

@@ -43,6 +43,7 @@ const socketPoolResource = async () => {
 const metricsResource = () => {
   const counterStub = { labels: () => ({ inc: () => null }) }
   const histogramStub = { labels: () => ({ observe: () => null }) }
+  const gaugueStub = { labels: () => ({ set: () => null }) }
   return {
     run: () => ({
       register: {
@@ -57,6 +58,9 @@ const metricsResource = () => {
       histograms: {
         MESSAGE_ROUNDTRIP: histogramStub,
       },
+      gauges: {
+        CHANNEL_HEALTH: gaugueStub,
+      },
     }),
   }
 }