Add tests and docs for first/last over time

Author: dnhatn

docs/reference/query-languages/esql/_snippets/functions/description/rate.md

@@ -18,3 +18,4 @@
 * [`WEIGHTED_AVG`](../../functions-operators/aggregation-functions.md#esql-weighted_avg)
 * [unavailable] [`FIRST_OVER_TIME`](../../functions-operators/aggregation-functions.md#esql-first_over_time)
 * [unavailable] [`LAST_OVER_TIME`](../../functions-operators/aggregation-functions.md#esql-last_over_time)
+* [unavailable] [`RATE`](../../functions-operators/aggregation-functions.md#esql-rate)

docs/reference/query-languages/esql/_snippets/functions/examples/rate.md

@@ -71,3 +71,6 @@ The [`STATS`](/reference/query-languages/esql/commands/processing-commands.md#es
 
 :::{include} ../_snippets/functions/layout/last_over_time.md
 :::
+
+:::{include} ../_snippets/functions/layout/rate.md
+:::