Define events for the vector sink

[jamtur01]

As part of finishing https://github.com/timberio/vector-product/issues/14, we need to define internal events for the vector sink.

Defining events

• If you have not already, please read RFC 2064.
  • Be sure to read the Guide-level Proposal section and the implementation details that affect performance.
• You can also look at the currently defined events for precedence.
• Events should be named properly: <ComponentName><Noun><Verb>. For example, the syslog source defines the SyslogEventReceived struct.

Requirements

• Add a InsertSinkNameHereEventProcessed event.
  • Derive an events_processed metric via the emit_metrics method. (example)
• Add a InsertSinkNameHereEventDiscarded event. This event only occurs if the event does not match the filter.
  • Derive an events_discarded metric via the emit_metrics method. (example)
• Audit and add events for all possible errors. (example)
  • Derive a processing_errors metrics via the emit_metrics method. (example)
  • Emit a log message that ends with ; discarding event only if the error results in dropping the event. (example.
• Audit and add events for all component-specific behavior. (example)
• Ensure that logs in the hot path are rate limited via the rate_limit_secs parameter. (example).
• Ensure that all log messages start with a capital letter and end with a .. (example)

[bruceg]

Replaced by #9325