Add metric for discarded lines to file source

[bruceg]

The file source increments a counter when it discards events (namely component_discarded_events_total). However, it may discard source data before they are translated into events. For example, if a source line is longer than max_line_bytes, it is discarded within the file-server lib and, while an warning log is emitted, no counter is incremented to provide evidence.

This condition should result in a counter being incremented, either component_errors_total with the notion that it represents malformed input data, or some other name if that is not appropriate.

Ref: no metric on File source on max_byte_line exceeded #14144

[bruceg]

Noting here that this should also be reflected in the kubernetes_logs source. Ref: #14144 (comment)