Add histograms for pool-ke session durations. #1105

Job	Run time
Clippy (aarch64-apple-darwin, macos-latest)	48s
Clippy (x86_64-unknown-linux-musl, ubuntu-latest)	48s
Build and test (stable, ubuntu-latest, x86_64-unknown-linux-gnu)	1m 20s
Clippy (armv7-unknown-linux-gnueabihf, ubuntu-latest)	37s
Build and test (msrv, ubuntu-latest, x86_64-unknown-linux-gnu)	1m 41s
Audit dependencies	25s
Format	6s
Clippy (x86_64-unknown-linux-gnu, ubuntu-latest)	40s
Fuzz with code coverage (client_request)	1m 31s
Fuzz with code coverage (record)	1m 8s
Fuzz with code coverage (key_exchange_response)	1m 19s
Fuzz with code coverage (server_information_response)	1m 12s
Build and test (beta, ubuntu-latest, x86_64-unknown-linux-gnu)	1m 16s
Build and test (stable, ubuntu-latest, x86_64-unknown-linux-musl)	1m 48s
Check unused dependencies	2m 7s
	16m 46s

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