[SPARK-25170][DOC] Add list and short description of Spark Executor Task Metrics to the documentation.

[LucaCanali]

What changes were proposed in this pull request?

Add description of Executor Task Metrics to the documentation.

[kiszk]

LGTM cc @jiangxb1987 @srowen

[srowen]

The docs looks OK. I guess these weren't documented before? I spot-checked the units on a few metrics and they're right. I guess it's a little unfortunate there are some ns and ms times in here, but not a problem.

[LucaCanali]

Thanks @srowen for reviewing this. The metrics are commented in the source code of TaskMetrics class, I took most of the descriptions from there, adding some additional explanations where needed. Indeed I agree that the fact that time units are not uniform is a bit incovenient.

[SparkQA]

Test build #4340 has finished for PR 22397 at commit 27130ba.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Merged to master/2.4

[kiszk]

Yes, time unit looks confusing, but not a problem. In general, due to APIs, elapsed time is ms, and CPU time is ns.