Fix b/304114403
- adds a new experimental tracer useful for diagnosing ping timeout
failures in unit tests
- adds a pair of experimental tracers for fuzzing event engine
- fix the behavior of FuzzingEventEngine so that a RunAfter(0, ...) runs
in the same tick
- up the rate of sends (reduce the send delay) so we guarantee to be
able to send 200kb/sec in fuzzed e2e unit tests
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Co-authored-by: ctiller <ctiller@users.noreply.github.com>