Remove time unit from rate aggregation (#126497)

Currently, the rate aggregation accepts two parameters: the first 
specifies the counter field, and the second specifies the time unit of 
the rate. The time unit parameter was introduced to conveniently compute 
requests per minute or per hour. However, this can be replaced easily -
for example, rate(field, 1minute) with 60 * rate(field).

This change removes the time unit parameter and reserves it for 
potential future usage, such as introducing a sliding window unit. If we 
decide on other options later, we can reintroduce it. Removing it now
avoids breaking changes while the rate aggregation is not yet available.

Author: dnhatn