### Describe Your Changes
Currently if multiple msgFields are present in a log row it's not
obvious which field is selected as a _msg field. With this PR and order
of msgfield values defined either via headers or query arg params
defines a priority of these values
### Checklist
The following checks are **mandatory**:
- [ ] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).