Watch command run benchmark tests for watch releated operations:
1. watch keys 2. sending events to watchers.
To test 2, this test also puts keys into etcd cluster to trigger
event sending.
Besides the benchmark results showed at client side, the tester
can also monitor the server-side mem/cpu usage and also check
the metrics of slow watchers. If there are a lot of slow watchers,
it means etcd server is over-loaded by watchers.
Xiang Li