Merge arbitrary metadata via all `Sibyl.Handlers.attach_*` functions.

[vereis]

It would be very useful to be able to attach arbitrary metadata which gets passed into all our handlers when attached, for custom metadata overrides such as setting a custom debug_user: :vereis or env: :staging.

[vereis]

Something like Sibyl.Handlers.attach_all_events(Sibyl.Handlers.OpenTelemetry, metadata: %{user: "[email protected]", reason: "debugging weird bug"})

Doing this would attach the above to all OpenTelemetry traces which could then be used for additional filtering on whatever OpenTelemetry compatible UI you use.