Track build time as a Metric

[epompeii]

Currently the runtime for Report is tracked however there is no built in way to track this like other Metrics.

Create a "Build Time" Measure that is collected.

One possibility is to use a --build-time flag / option with an optional, explicit override.
This will also need to play nice with the File Size option: #383

Therefore, --build-time could be used with or without a benchmark command.
But it would conflict with the --file option, just as --file-size does.

The open question is whether to require --file-size and if not what to use as the benchmark name, the benchmark command?

Alternatively, it could turn out that like benchmark harness results getting the metric itself is more than half the battle. In this case, there may need to be "build time adapters".