Convert metric aggs docs runtime fields

[nik9000]

This replaces the script docs for bucket aggregations with runtime
fields. We expect runtime fields to be nicer to work with because you
can also fetch them or filter on them. We expect them to be faster
because their don't need this sort of instanceof tree:

elasticsearch/server/src/main/java/org/elasticsearch/search/aggregations/support/values/ScriptDoubleValues.java

Line 42 in a92a647

} else if (value instanceof Number) {

Relates to #69291

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[not-napoleon]

LGTM. Thanks for jumping on this.

[jrodewig]

LGTM. I left a few suggestions to avoid second person (mostly "we") wording. Those are non-blocking, and you can ignore them if wanted. Thanks for handling this @nik9000.

[lockewritesdocs]

Started a review with non-blocking changes while James was reviewing, so might be some overlap.

[lockewritesdocs]

Suggested change

	If you need to create a boxplot for values that aren't indexed exactly you
	should create a <<runtime,runtime field>> and get the boxplot of that. For
	example, if our load times are in milliseconds but we want values calculated
	in seconds, we could use a runtime field to convert them on-the-fly:
	If you need to create a boxplot for values that differ from the indexed values,
	create a <<runtime,runtime field>> and then compute the boxplot. For
	example, if your load times are in milliseconds but you want values calculated
	in seconds, use a runtime field to convert values within a query:

[lockewritesdocs]

I know that "on-the-fly" was in these docs before you made changes, but we try to avoid idioms like these because they don't translate well beyond English.

[nik9000]

👍

[nik9000]

Thanks for the reviews folks!