Feature request(AIP-192): Add a linter check for missing backticks

See the code snippet below which has a missing backtick. This can cause issue in client library documentation and a linter rule would help prevent issues from being introduced upstream.

```
// `"application/json"`. The default value is set to "application/json"`.
```
should be
```
// `"application/json"`. The default value is set to `"application/json"`.
```

https://github.com/googleapis/googleapis/blob/674883e3c55a16ed558ea0df1125537a956ff945/google/cloud/tasks/v2beta3/target.proto#L196