average time per build step

[trey-ivy]

today on the trends view we track an overal invocation time but this is generally worthless. much better would be a view that allows you to select which paramters you want to view for analysis. The most useful ones I can think of would be a per-ci-build-step differntiation. i don't want to get into the business of building graphs and visualizations, i'd rather provide a mechanism to dump the data to grafana (or similar) and let users roll their own dashboards, but a few basic at a glance visualizations may provide some helpful functionality, and something to allow you to view invocation times per step and/or per worker might be nice