Add StartedQueries stat to InternalResourceGroup for tracking query start rate

[xkrogen]

Description

Add a new StartedQueries stat to InternalResourceGroup to track the rate of queries being started/submitted to a resource group. After #22957 we have nearly all of the useful operational stats that you would want to know about a resource group (like CPU / memory, queued/running queries at any given time, etc.), but one thing that is missing is an understanding of the rate at which queries are being submitted into this resource group.

Additional context and related issues

Currently we have a metric TimeBetweenStartsSec but it doesn't meet the needs described here:

1. It only tracks time between starts, not the count of queries
2. It doesn't count the first start after newly becoming eligible (refer to this discussion), limiting its utility for tracking the query submission/start rate.
3. It currently appears to be broken in two ways; first, it only updates for non-leaf groups (refer to this Slack discussion); second, it is a CounterStat, but is trying to measure timing information, so it really should be a TimeStat. Currently it is putting timing information (count of seconds) into a CounterStat which doesn't really produce meaningful data.

Point (3) is fixable and I have a WIP to address it (xkrogen@c2d474d), but after working on it I have become convinced that even if fixed, it is confusing and not useful. I am more inclined to just remove it and entirely replace it with this new metric. Will be happy to do that as a follow-on to this PR if others agree, or if there is consensus that TimeBetweenStartsSec indeed adds significant value and it's worth fixing the issues described in (3), I can finish up my WIP changes.

Release notes

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# General
* Expose query start count/rate metrics for resource groups. ({issue}`23984`)

[xkrogen]

cc @piotrrzysko @hashhar