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

- Stack version: 8.19.21-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-57752 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/49027

Latest failed builds:
- https://buildkite.com/elastic/integrations/builds/49093
- https://buildkite.com/elastic/integrations/builds/49169
- https://buildkite.com/elastic/integrations/builds/49242
- https://buildkite.com/elastic/integrations/builds/49336

Start with the mongodb package's collstats DataStream and the `system test: collstats` failure, then inspect the failed Buildkite runs for the first occurrence. Trace why `mongodb.collstats.stats.capped`, `freeStorageSize`, and `scaleFactor` are undefined; done means the system test passes without missing-field errors.

Written by the indexing model from the issue text.