Enable automated tests for `TS` command, `rate` and `{agg}_over_time` functions

[kkrik-es]

We need to add documentation for TS command, per instructions.

We also need to add automated tests for time-series aggregate functions:

• rate
• avg_over_time
• count_over_time
• count_distinct_over_time
• min_over_time
• max_over_time
• sum_over_time
• first_over_time
• last_over_time

We probably want to introduce and use a subclass of AbstractAggregationTestCase that will mark these functions as time-series aggregates - see also AbstractAggregationTestCase::isAggregation.

[elasticsearchmachine]

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

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)