chore: release 1.6.0 (#330)

Author: release-please[bot]

monitoring/snippets/package.json

@@ -14,7 +14,7 @@
     "test": "mocha --timeout 600000"
   },
   "dependencies": {
-    "@google-cloud/monitoring": "^1.5.0",
+    "@google-cloud/monitoring": "^1.6.0",
     "yargs": "^14.0.0"
   },
   "devDependencies": {