Make latency measurement in ping message robust against time drift #679

Job	Run time
docs	1m 9s
Clippy	55s
benches	3m 20s
rustfmt	4s
Run the default tests	2m 53s
Test ractor without async-trait	3m 3s
Test ractor without span propogation	2m 53s
Test ractor with the `cluster` feature	2m 52s
Test ractor with the `blanket_serde` feature	2m 56s
Test ractor_cluster with native async traits	2m 28s
Test ractor_cluster with async_trait	2m 40s
Test ractor with the monitor API	2m 55s
	28m 8s

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