[LogsDB] [Stack 8.19.21-SNAPSHOT] [mongodb] Failing test daily: system test: collstats in mongodb.collstats

- Stack version: 8.19.21-SNAPSHOT
- LogsDB: enabled
- 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-38983 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/49028

Latest failed builds:
- https://buildkite.com/elastic/integrations/builds/49092
- https://buildkite.com/elastic/integrations/builds/49170
- https://buildkite.com/elastic/integrations/builds/49243
- https://buildkite.com/elastic/integrations/builds/49335

Start by locating the MongoDB integration's collstats data stream and its system test, then 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 without undefined-field errors.

Written by the indexing model from the issue text.