I've seen two recent non-deterministic failures:
1. After starting the EventListener there is a delay of one collection
interval (currently 0.3 seconds) where we expect calls to counter.Add
and Histogram.Record() to update stats. On a fast machine this code
would run in under a millisecond but in some test runs it still wasn't
happening fast enough.
2. We were seeing error events from a previous test get observed in a
later test because session id was being ignored for error events.

Fixes:
1. Added an initial collection interval where no counter APIs will be
invoked and the test will delay up to 5 seconds waiting for this.
Hopefully this delay makes it more likely that the Add/Record APIs
are ready to execute promptly once the 2nd interval begins. If that
still isn't sufficient we can either increase the collection intervals or
disable the tests.
2. Tightened up the session id matching so only error events with
empty id or the expected id are allowed.