influxdb_listener doesn't respect the db in the query string

[morfien101]

When sending metrics via the influxdb_listener allows you to send the database in the query string, however the plugin does not respect it.

Use case:
We would like to proxy our requests that our developers send directly to Influx to allow us to add and black list tags or fields.
We don't really want to have the development teams change how they send metrics. We control the location via environment variables.

Expected behaviour:
When a database is sent in the query it should be added as a database tag unless there is one already. This allows downstream outputs to send the metrics to the correct databases. Given this is a InfluxDB proxy, I would expect this to be the correct way or handling requests.

I'm happy to submit a PR for this feature/bug if it is accepted as a problem. If not can I be told what I should rather be doing to achieve this behaviour.

[danielnelson]

Added in #6257