httpjson: add event publishing metrics

[andrewkroh]

Describe the enhancement:

The CEL input provides metrics (docs) about the number of events published and some details about the amount of time it took to publish the data (which gives a signal about amount of time to ACK). For example, CEL has metrics like

Metric	Description
batches_published_total	Number of event arrays published.
events_published_total	Number of events published.
batch_processing_time	Histogram of the elapsed successful batch processing times in nanoseconds (time of receipt to time of ACK for non-empty batches).

I would like to have similar metrics available for httpjson.

Describe a specific use case for the enhancement or feature:

• In diagnostic dumps from Agent in input_metrics.json, I would like to have a metric that shows the total number of events published. Batch stats would be useful too.

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)