feat(outputs.influxdb_v2): report total amount of bytes sent, number of retries, number of failures

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### Use Case

We are interested in seeing how much data we send over the satellite exactly. Right now, we have a rough estimate, but we would like to see this more precisely. Even nicer would be per bucket/address.

I propose to add a selfstat counter that keep track of how much data was sent.

Second, some counters should also be added to keep track of failures and retries

### Expected behavior

There is a metric `internal_influxdb2,bucket=destination,url=http://localhost:8086 bytes_sent` that reports how many bytes of data have been sent to the destination, per bucket and per url. This counter also includes any failed writes.

### Actual behavior

Not available

### Additional info

_No response_

Start by locating the outputs.influxdb_v2 plugin and its existing selfstat instrumentation. Trace how writes are assembled and sent, then review any nearby tests before defining coverage for bytes sent, retries, and failures, including bucket and URL labels. Done means the requested counters are reported and failed writes are included in bytes_sent.

Written by the indexing model from the issue text.