Allow debug setting override per plugin instance

## Feature Request

Opening a feature request kicks off a discussion.

### Proposal:

Allow debug setting override per plugin instance.

### Current behavior:

Debug setting is global and affects all plugins.

### Desired behavior:

Ability to override the default per input instance (debug = true | false)

### Use case:

There's some debug output (like output.influxdb buffer fulness and batch write metrics) that is useful to keep enabled.
If the global is enabled, suddenly a change to a plugin may cause massive amount of debug messages being output, and the only possibility is to disable the setting globally. With this feature, we can chose to silence just one plugin, or enforce debug for just one plugin (for example)

Start by tracing how the global debug setting reaches plugin instances; the issue does not name specific files, tests, or entry points. Done means each input instance can override the default with debug=true or false, while existing global behavior remains available and plugin-specific debug output can be enabled or silenced independently.

Written by the indexing model from the issue text.