Make latency measurement in ping message robust against time drift #676

Job	Run time
benches	3m 19s
Clippy	53s
docs	1m 10s
rustfmt	5s
Run the default tests	2m 53s
Test ractor without async-trait	2m 58s
Test ractor without span propogation	3m 0s
Test ractor with the `cluster` feature	3m 1s
Test ractor with the `blanket_serde` feature	2m 59s
Test ractor_cluster with native async traits	2m 30s
Test ractor_cluster with async_trait	2m 37s
Test ractor with the monitor API	3m 2s
	28m 27s

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