Non-stringy label values in MetricFamily metrics cause export-time errors

This is very similar to #18 only I'm using the GaugeMetricFamily/CounterMetricFamily interface, otherwise the symptoms are the same. I'm using the default pypi version (which I think is 0.2.0).

I was inadvertently passing in ints for a couple of the label values in .add_metric(), which results in a 500 error on export and the following traceback. Wrapping a str() around the values makes this go away. Coercing these values as was done in the fix for #18 seems like the right thing to do here as well.

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/socketserver.py", line 639, in process_request_thread
    self.finish_request(request, client_address)
  File "/usr/local/lib/python3.6/socketserver.py", line 361, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/local/lib/python3.6/socketserver.py", line 696, in __init__
    self.handle()
  File "/usr/local/lib/python3.6/http/server.py", line 418, in handle
    self.handle_one_request()
  File "/usr/local/lib/python3.6/http/server.py", line 406, in handle_one_request
    method()
  File "/usr/local/lib/python3.6/site-packages/prometheus_client/exposition.py", line 95, in do_GET
    output = generate_latest(registry)
  File "/usr/local/lib/python3.6/site-packages/prometheus_client/exposition.py", line 78, in generate_latest
    for k, v in sorted(labels.items())]))
  File "/usr/local/lib/python3.6/site-packages/prometheus_client/exposition.py", line 78, in <listcomp>
    for k, v in sorted(labels.items())]))
AttributeError: 'int' object has no attribute 'replace'

Comience con GaugeMetricFamily y CounterMetricFamily, siguiendo .add_metric() a través de generate_latest en exposition.py hasta donde aparece el traceback. Verifique que los valores enteros de las etiquetas se acepten durante la exportación sin el error de replace(), preservando al mismo tiempo el comportamiento existente descrito para issue #18.