Internal plugin additional fields

### Proposal:
Add metrics_dropped and metrics_filtered fields to input plugins.
Add metrics_dropped, metrics_filtered, metrics_gathered(processed/aggregated) to processors and aggregators plugins.

### Current behavior:
metrics_dropped and metrics_filtered are only available for output plugins.
For processors and aggregators plugins the only available field is internal_process_errors

### Desired behavior:
Being able to see how many metrics has been filtered/dropped out on the input level.
Being able to see how many metrics has been gathered(processed/aggregated)/filtered/dropped on the processor/aggregator level.

### Use case:
Improving telegraf self monitoring, additional performance measurement, could be very useful with exec input plugin or regexp processor plugin

Start by locating how internal metrics are exposed for output plugins, then compare the input, processor, and aggregator plugin paths. Trace where dropped, filtered, gathered, processed, and aggregated counts could be reported. Done means the requested fields are available at the stated plugin levels and their values can be verified through the existing self-monitoring output.

Written by the indexing model from the issue text.