Use getOrDefault in IngestDocument rather than containsKey+get

[joegallo]

This is half a code tidiness change (it irks me every time I see in these code paths that we're doing this), and half a performance improvement (it irks me that we're doing it because it is slower).

Consider that basically every ingest processor gets a value from the document, does something to that value, and then sets the value somewhere in the document. On the get side and on the set side, we're paying a map lookup cost twice that we could just be paying once.

In the case of the map in question being a CtxMap (which it pretty much always is for the first key of a path traversal), we were also paying the cost of calling metadata.isAvailable(str) twice, too.

Note that this same trick was already applied to to Metadata itself in a previous PR (specifically #93333).

Anyway, this doesn't make an enormous different on any one processor or pipeline, but it speeds everything up just a little. For example, microbenchmarking on my machine, it speeds up an example rename processor by 20%, but that's precisely because a rename processor is so fast that the cost of the wasted map lookups actually matters. On a real workload this is probably a modest one or two percent improvement in overall ingest processing time, but that would depend on the workload, of course.

Here's some screenshots:

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[nielsbauman]

LGTM!

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x