Additional ideas for performance counters

[roji]

#18016 covers the basic counters implemented for EF Core 5.0. Below are some additional ideas that we may implement in the future, based on user feedback:

• Initialized DbContexts (total/rate per second)
• Relational command cache hit/miss ratio. This would also be our only relational counter, and the value here is questionable.
• Batching statistics (average DbCommands per batch). Not clear how this can be actionable.
• DbConnection statistics (e.g. connection that are currently open, or number of connections with counter > 0)
• General/basic execution statistics (how many commands are getting executed, etc.). This would probably largely duplicate what the underlying ADO.NET provider could/would expose, but it's still valuable (for ADO.NET providers which don't yet have event counters, for non-relational providers).
• Lazy loading metrics. Not immediately clear how to understand identify the N+1 problem from this number.
• Time spent "in queries"
• Specific statistics for tagged queries (see proposal by @unaizorrilla in Expose event counters for performance metrics #18016 (comment)), possibly under a separate EventSource (e.g. EFCore.TaggedQueries)