Currently REST mode transcode doesn't support non-alphanumeric in the field name, and it cause [firestore test](https://github.com/googleapis/nodejs-firestore/blob/main/dev/system-test/firestore.ts#L754-L771) fails.

code sample:
```
test() {
    const ref = randomCol.doc('doc');
    return ref
      .set({'!.\\`': {'!.\\`': 'value'}})
      .then(() => {
        return ref.get();
      })
      .then(doc => {
        // SEE ERROR ON THIS LINE
        expect(doc.data()).to.deep.equal({'!.\\`': {'!.\\`': 'value'}});
        return ref.update(new 
FieldPath('!.\\`', '!.\\`'), 'new-value');
      })
      .then(() => {
        return ref.get();
      })
      .then(doc => {
        expect(doc.data()).to.deep.equal({'!.\\`': {'!.\\`': 'new-value'}});
      });
}
```

Error
```
AssertionError: expected { '.': { '.': 'value' } } to deeply equal { '!.\\`': { '!.\\`': 'value' } }
```

Source-Link: googleapis/synthtool@89dd35d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:89c5b2f3decec8ad64febbebea671076c119d1ab43700da380846a315600de8a

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Summer Ji <[email protected]>

🤖 I have created a release *beep* *boop*
---


### [2.29.4](v2.29.3...v2.29.4) (2022-01-19)


### Bug Fixes

* support non-alphanumeric field name ([#1165](#1165)) ([4f53efa](4f53efa))

---
This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please).