[alerting event log] buffer events being written instead of writing when logged

[pmuellr]

Currently, event log entries are written to the es index when clients make the call to log an event.

Instead, we should buffer these events, and do a bulk index (write) of them, as needed.

As needed probably means:

• the buffer is getting full (eg, buffer of 100 entries) OR
• a time limit elapses (eg, 5 seconds)

There is already an initial take on a "bounded queue", written to deal with this: https://github.com/elastic/kibana/blob/master/x-pack/plugins/event_log/server/lib/bounded_queue.ts

Having these buffered solves a couple of issues:

• changes from constant, small index requests to ES, to more occasional bulk index request; presumably performing a bit better
• handles some cases of ES not being available; if the write fails, we can keep the buffer around, and try again later, in hopes that ES comes back up

One other note I've been thinking of. We probably want to assign id's for the event log entries as uuid's, or other unique keys we can generate in the plugin. If we can do that, since the event_log is append only, if the same event gets written twice, for some reason, the second would just be overwriting the first, rather than appearing as a clone entry of the first.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[pmuellr]

Was just poking through rxjs, and noticed a handy bufferTime() operator, that will buffer up to a max count or elapsed time period, which would be perfect for this use case.

We'll likely want to buffer for 1 to some small number of seconds, for maybe 100 events max. We can then do a bulk index of those events.