Error handling breaks when using rest fallback

We're currently creating our `MetricServiceClient` using the rest fallback method, since we're building our code with esbuild and using protos does not seem to be supported. However, if the `createTimeSeries` call fails, the the error is not handled and emitted by the library. Instead, it causes a crash and an unhelpful error message is emitted.

Stacktrace:
```
⚠ functions: Error: googleProtobufAnyFromProto3JSON: cannot find type google.monitoring.v3.CreateTimeSeriesSummary: Error: no such type: google.monitoring.v3.CreateTimeSeriesSummary
at googleProtobufAnyFromProto3JSON (/poggio/node_modules/proto3-json-serializer/build/src/any.js:70:15)
at fromProto3JSONToInternalRepresentation (/poggio/node_modules/proto3-json-serializer/build/src/fromproto3json.js:58:58)
at /poggio/node_modules/proto3-json-serializer/build/src/fromproto3json.js:105:48
at Array.map ()
at fromProto3JSONToInternalRepresentation (/poggio/node_modules/proto3-json-serializer/build/src/fromproto3json.js:105:33)
at Object.fromProto3JSON (/poggio/node_modules/proto3-json-serializer/build/src/fromproto3json.js:149:26)
at GoogleErrorDecoder.decodeHTTPError (/poggio/node_modules/@google-cloud/monitoring/node_modules/google-gax/build/src/googleError.js:198:41)
at Function.parseHttpError (/poggio/node_modules/@google-cloud/monitoring/node_modules/google-gax/build/src/googleError.js:71:37)
at decodeResponse (/poggio/node_modules/@google-cloud/monitoring/node_modules/google-gax/build/src/fallbackRest.js:69:49)
at /poggio/node_modules/@google-cloud/monitoring/node_modules/google-gax/build/src/fallbackServiceStub.js:101:42
⚠ Your function was killed because it raised an unhandled error.
```

This is roughly how we're using the library:

```
const client = new MetricServiceClient({ fallback: "rest" });
const dataPoint = {
interval: {
endTime: {
seconds: Date.now() / 1000,
},
},
value: {
int64Value: 1,
},
};

const requestArgs = {
name: metricServiceClient().projectPath('project-name'),
timeSeries: [
{
metric: {
type: 'custom.googleapis.com/connectors/fivetran_sync_start',
metricKind: 'GAUGE',
valueType: 'INT64',
labels: [
runId: 'abc1234'
],
},
resource: {
type: "global",
labels: {
project_id: 'project-name',
},
},
points: [dataPoint],
},
],
};
await metricServiceClient().createTimeSeries(requestArgs);
```

#### Environment details

- OS: Ubuntu 18.04
- Node.js version: 16
- npm version:
- `@google-cloud/monitoring` version: 3.0.2

#### Steps to reproduce

1. Create a MetricServiceClient with the rest fallback option, e.g. `new MetricServiceClient({ fallback: "rest" })`
2. Do something that causes `createTimeSeries` to error (in our case, I believe we were exceeding the rate limit).
3. Instead of an error message (which is what you get if you use the proto fallback), you get the following error: `Error: googleProtobufAnyFromProto3JSON: cannot find type google.monitoring.v3.CreateTimeSeriesSummary: Error: no such type: google.monitoring.v3.CreateTimeSeriesSummary`.