Fix inaccurate per-block metrics (#6491)

`Measure` incorrectly assumed that it was the only thing being done by the parent `LoggingContext`. For instance, during a "renew group attestations" operation, hundreds of `outbound_request` calls could take place in parallel, all using the same `LoggingContext`. This would mean that any resources used during *any* of those calls would be reported against *all* of them, producing wildly inaccurate results.

Instead, we now give each `Measure` block its own `LoggingContext` (using the parent `LoggingContext` mechanism to ensure that the log lines look correct and that the metrics are ultimately propogated to the top level for reporting against requests/backgrond tasks).