refactor(consumption_metrics): prereq refactorings, tests (#5315)

Split off from #5297.

There should be no functional changes here:
- refactor tenant metric "production" like previously timeline, allows
unit testing, though not interesting enough yet to test
- introduce type aliases for tuples
- extra refactoring for `collect`, was initially thinking it was useful
but will do a inline later
- shorter binding names
- support for future allocation reuse quests with IdempotencyKey
- move code out of tokio::select to make it rustfmt-able
- generification, allow later replacement of `&'static str` with enum
- add tests that assert sent event contents exactly

proxy/src/metrics.rs

@@ -121,7 +121,7 @@ async fn collect_metrics_iteration(
 
     let current_metrics = gather_proxy_io_bytes_per_client();
 
-    let metrics_to_send: Vec<Event<Ids>> = current_metrics
+    let metrics_to_send: Vec<Event<Ids, &'static str>> = current_metrics
         .iter()
         .filter_map(|(curr_key, (curr_val, curr_time))| {
             let mut start_time = *curr_time;