Add monitor API back (#317) #674

Job	Run time
Clippy	1m 4s
docs	1m 10s
rustfmt	9s
Run the default tests	2m 59s
benches	3m 17s
Test ractor without async-trait	3m 3s
Test ractor without span propogation	3m 16s
Test ractor with the `cluster` feature	2m 57s
Test ractor with the `blanket_serde` feature	2m 56s
Test ractor_cluster with native async traits	2m 33s
Test ractor_cluster with async_trait	2m 31s
Test ractor with the monitor API	2m 55s
	28m 50s

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