Add aggregate specific metrics

### Is your feature request related to a problem or challenge?

Yes, running a query that takes a long time in the aggregator, you don't know what is taking the time there, which expression for example or what inside the expression

in our case we have aggregate with 2 aggregate expression that are the same (just on different column), in that aggregate expression impl there are multiple phases that can contribute to the timing

### Describe the solution you'd like

having metrics per aggregate expression with the aggregate specific metric that can be declared will make it easier for visibility

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by tracing the aggregator implementation and its existing timing or metrics instrumentation. Identify the phases within aggregate expression execution and determine how metrics could be exposed per expression; done means users can distinguish timing contributions from separate aggregate expressions and their phases.

Written by the indexing model from the issue text.