Use prometheus as a metric backend

This entails writing a metric aggregation program since each worker has its
own memory, and thus own metrics because of python gunicorn. The python
client is a simple wrapper that makes web requests to it.

buildman/server.py

@@ -17,8 +17,11 @@ from buildman.jobutil.buildstatus import StatusHandler
 from buildman.jobutil.buildjob import BuildJob, BuildJobLoadException
 from data import database
 from app import app, metric_queue
+from app import app
+from util.prometheus import Counter
 
 logger = logging.getLogger(__name__)
+build_counter = Counter('builds', 'Number of builds', labelnames=['name'])
 
 WORK_CHECK_TIMEOUT = 10
 TIMEOUT_PERIOD_MINUTES = 20
@@ -238,3 +241,4 @@ def report_completion_status(status):
     return
 
   metric_queue.put(status_name, 1, unit='Count')
+  build_counter.Inc(labelvalues=[status_name])