[Stack 8.19.20-SNAPSHOT] [mongodb] Failing test daily: system test: collstats in mongodb.collstats

- Stack version: 8.19.20-SNAPSHOT
- Package: mongodb
- Failing test: system test: collstats
- DataStream: collstats
- Owners:
- @elastic/obs-infraobs-integrations

Failure:
```
test case failed: one or more errors found in documents stored in metrics-mongodb.collstats-31918 data stream: [0] field "mongodb.collstats.stats.capped" is undefined
[1] field "mongodb.collstats.stats.freeStorageSize" is undefined
[2] field "mongodb.collstats.stats.scaleFactor" is undefined
```

First build failed: https://buildkite.com/elastic/integrations/builds/47075

Latest 5 failed builds:
- https://buildkite.com/elastic/integrations/builds/48319
- https://buildkite.com/elastic/integrations/builds/48398
- https://buildkite.com/elastic/integrations/builds/48455
- https://buildkite.com/elastic/integrations/builds/48532
- https://buildkite.com/elastic/integrations/builds/48614

Start with the mongodb package's system test for the collstats data stream and reproduce the failure reported in the linked Buildkite runs. Trace why mongodb.collstats.stats.capped, freeStorageSize, and scaleFactor are undefined. Done means the collstats system test passes and the metrics-mongodb.collstats data stream contains the expected fields.

Written by the indexing model from the issue text.