quinn: export endpoint::EndpointStats #4714

Job	Run time
msrv	34s
lint	57s
audit	22s
test (ubuntu-latest, stable)	1m 19s
test-android (x86_64-linux-android, x86_64, 26)	4m 45s
test (ubuntu-latest, beta)	1m 15s
test on freebsd	4m 32s
test-android (i686-linux-android, x86, 26)	3m 37s
test (macos-latest, stable)	1m 4s
test (windows-latest, stable)	3m 25s
features (ubuntu-latest)	5m 18s
features (macos-latest)	4m 1s
features (windows-latest)	8m 57s
	40m 6s

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