use processors to collect build metrics.

def process_build_metrics(context, build_processors):
    """use processors to collect build metrics."""
    build_metrics = OrderedDict()

    # reset all processors
    for p in build_processors:
        p.reset()

    # collect metrics from all processors
    for p in build_processors:
        build_metrics.update(p.build_metrics)

    return build_metrics