outputs.influxdb_v2: Expose bucket in Telegraf self metrics

### Use Case

When monitoring Telegraf with Prometheus, there's currently no way to tell which InfluxDB bucket an `outputs.influxdb_v2` instance is writing to.

It would be really useful if Telegraf's self metrics exposed the configured bucket, along with some basic information about what is being written (for example, measurement statistics).

This would make it much easier to verify that a Telegraf instance is writing to the expected bucket and to troubleshoot missing or unexpected data, without having to inspect the configuration or query InfluxDB directly. It also helps to see in what buckets the data can be queried from for each telegraf instance.

### Expected behavior

Telegraf's self metrics should expose the configured InfluxDB bucket (and optionally basic information about what is being written), so Prometheus users can identify where each outputs.influxdb_v2 instance is sending data.

### Actual behavior

Telegraf's self metrics expose write statistics for the outputs.influxdb_v2 plugin, but they don't indicate which InfluxDB bucket the output is writing to. When monitoring Telegraf with Prometheus, there's no way to see which bucket an instance is targeting.

### Additional info

_No response_

Start by locating the outputs.influxdb_v2 entry point and its existing self metrics for output write statistics. Review how those metrics identify output instances, then consider the expected done state: Prometheus-visible metrics should identify the configured bucket for each instance; clarify whether measurement statistics are in scope.

Written by the indexing model from the issue text.