Metrics are dropped silently when metric buffer is full

[danielnelson]

Relevant telegraf.conf:

System info:

[Include Telegraf version, operating system name, and other relevant details]

Steps to reproduce:

1. Setup an output that can Connect() but not Write(), or otherwise arrange for metrics to be dropped.
2. Set a small metric buffer to reduce wait time.

Expected behavior:

Periodic update, perhaps before writing:

E! [outputs.file] %d metrics have been dropped since last flush

Actual behavior:

No notification unless using the internal plugin.

Additional info: