ref(similarity): add more tracing to grouping logic

[JoshFerge]

As i was debugging #inc-1175 I noticed we could use some more spans in our grouping logic.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: src/sentry/event_manager.py

Function	Unhandled Issue
assign_event_to_group	Group.DoesNotExist: Group matching query does not exist. sentry.tasks.store.sa... Event Count: 3

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