The overarching intent of the Build API Metrics subsystem is to enable exploration, and eventually monitoring and alerting for build-related performance issues.
The Build API Metrics subsystem is defined in terms of Metric Events. See infra/proto/src/chromiumos/metrics.proto.
During an invocation of the Build API, we set up an environment variable BUILD_API_METRICS_LOG which specifies a log file into which metric events can be appended. Afterwards, the Build API can call into the utils/metrics.py code to read the metric events out (see chromite.api.deserialize_metrics_log.) This data is transformed into MetricEvents (as per metrics.proto). These are fed back through the Build API's Response message which is returned to the recipes code. There it is uploaded into BigTable for later exploration using various front-ends, such as Dremel.
