[Feature Request] Write to dynamic collection in MongoDB, with the Apache Beam SDK in Python:

### What would you like to happen?

I have a query with the coll parameter in apache_beam.io.WriteToMongoDB

```
Pcoll
| "Write to Mongo" >> apache_beam.io.WriteToMongoDB(
uri='someUri',
db='someDb',
coll='someColl',
batch_size=10
)
```

I would like to like to set the collection name dynamically based on the record, instead of hard coding it. For ex. if the record is dict. and if it has something like record.tag = 'example', I would like to retreive that and set as coll.

I got some similar references for BigQuery and tried coll = lambda record: record.tag, which doesn't work. It threw an error saying it needs a string.

### Issue Priority

Priority: 2

### Issue Component

Component: io-py-mongodb