outputs(wavefront): inline retry logic may log `"Non-retryable error during Wavefront.Write: <nil>"`

Nobody has claimed this yet.

I'm noticed log lines like `Non-retryable error during Wavefront.Write: ` in Telegraf v1.28.2.

There's a code path in `(*Wavefront).Write` where `err` can be reassigned to `nil` on line 154 if a second call to `w.sender.SendMetric` succeeds. If the 2nd attempt does not return an error, we don't want to log anything about an error.

https://github.com/influxdata/telegraf/blame/f4c56e1597956cd95eb37336e0c08ae361338aba/plugins/outputs/wavefront/wavefront.go#L146

The retry/flush logic has gotten complicated in this method. I'm a little uncertain about the flushing and returning on line 150, and about line 157 returning an error mid-batch.

I think the mid-batch error causes Telegraf to retry the entire batch on the next Write, and I think that may mean successfully sent metrics, unsendable metrics, and unsent metrics all get retried.

Start in plugins/outputs/wavefront/wavefront.go, especially (*Wavefront).Write around lines 146-157, and trace the calls to w.sender.SendMetric. Check how the second attempt, flushing, and mid-batch returns affect retries. Done means a successful retry does not log a nil error and the resulting batch behavior is verified.

Written by the indexing model from the issue text.