build: add generated-files bot config (#989)

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Synth log will be available here:
https://source.cloud.google.com/results/invocations/74f0c42b-d8fe-4430-b878-ab099e9e7fe7/targets

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Source-Link: googleapis/synthtool@e6f3d54
Source-Link: googleapis/synthtool@04573fd

.github/generated-files-bot.yml

@@ -0,0 +1,13 @@
+generatedFiles:
+- path: '.kokoro/**'
+  message: '`.kokoro` files are templated and should be updated in [`synthtool`](https://github.com/googleapis/synthtool)'
+- path: '.github/CODEOWNERS'
+  message: 'CODEOWNERS should instead be modified via the `codeowner_team` property in .repo-metadata.json'
+- path: '.github/workflows/**'
+  message: '`.github/workflows` (GitHub Actions) should be updated in [`synthtool`](https://github.com/googleapis/synthtool)'
+- path: '.github/generated-files-bot.+(yml|yaml)'
+  message: '`.github/generated-files-bot.(yml|yaml)` should be updated in [`synthtool`](https://github.com/googleapis/synthtool)'
+- path: 'README.md'
+  message: '`README.md` is managed by [`synthtool`](https://github.com/googleapis/synthtool). However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/master/.readme-partials.yaml'
+- path: 'samples/README.md'
+  message: '`samples/README.md` is managed by [`synthtool`](https://github.com/googleapis/synthtool). However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/master/.readme-partials.yaml'

.github/workflows/ci.yaml

@@ -1,7 +1,7 @@
 on:
   push:
     branches:
-      - $default-branch
+      - master
   pull_request:
 name: ci
 jobs:

synth.metadata

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/gax-nodejs.git",
-        "sha": "1e15e2786b482630988cdd09705726f342e626d8"
+        "sha": "d4e495af46176b8dd74595108f25a6df99ee042d"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "c6706ee5d693e9ae5967614170732646590d8374"
+        "sha": "e6f3d54be015a394b6ab5a25903ec09062a2b424"
       }
     }
   ]