Add benchmark for OnLogEvent flow.

Benchmark uses a config with one Event Metric that receives events and 1000 additional
event metrics that don't receive any events and are only there to better
mimic actual scenarios where statsd has more metrics and matchers that
take part in matcher evaluation.

Bug: 318405610
Test: statsd_benchmark
Change-Id: Ic27e07ff6cbec82e6551c7f529f836a8c546906d
2 files changed