Incorrect error count metrics when using buffer in output plugin

Check [CONTRIBUTING guideline](https://github.com/fluent/fluentd/blob/master/CONTRIBUTING.md) first and here is the list to help us investigate the problem.

**Describe the bug**
When using a buffer, the num_errors reports all errors occurred, even if some of them are resolved by a retry.

For example, when using GCS output plugin, every error will be reported - even though this is solved after a couple of retries.

Looking at the [code](https://github.com/fluent/fluentd/blob/36d017deb6b37a55833691edbab8d5f31353ac36/lib/fluent/plugin/output.rb#L817), I noticed there is already a TODO to fix that so I guess this is a known issue. I'll be happy to help fixing it - just need some help to discuss the solution :)

**To Reproduce**
1. Create output plugin that produce random error and a buffer
2. Use it and emit events
3. Notice the `num_errors` is increased, even thought the retry eventually solves the error and the event are sent.

**Expected behavior**
Move errors to a different counter and report an error only when reaching the retry limit.