Exclude _msg field from query builder

[shxpd]

The query builder in the VictoriaLogs Grafana datasource plugin offers autocomplete suggestions for field values using the field_values API. This works really well for low-cardinality fields like host, level, app, namespace or any other stream field.

However, the _msg field is also available for value suggestions, which is problematic:

• _msg contains the actual log message content and is inherently high-cardinality
• In real-world deployments with hundreds of applications, each producing hundreds of unique log lines, there could be millions of unique _msg values
• Requesting field_values for _msg is slow - scanning millions of unique values, memory-intensive and pretty useless - nobody will scroll through millions of log messages to pick one.

I'd like to have exclude _msg from field value suggestions entirely. Or add it to a configurable exclude list.

[arturminchukov]

@shxpd thanks for you imrovement request. Right now, we have an open PR related to a new query builder. But there are some bugs. I don't have the ETA for when it will be merged. But when I have more info about it, I will take your issue in progress.