Non-string label values cause export-time failures

[korfuri]

Label values are not checked for string-likeness, and are not explicitly converted to strings. This causes the following problems:

c = Counter('http_requests_total_by_method', 'Count of requests by HTTP method.', ['method']

def handle_request(request):
  c.labels(request.method).inc()

to fail silently if request.method is None (or a tuple, or an object, etc.). At export time, the following will happen:

Exception happened during processing of request from ('127.0.0.1', 55703)
Traceback (most recent call last):
  File "/usr/lib/python2.7/SocketServer.py", line 295, in _handle_request_noblock
    self.process_request(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 321, in process_request
    self.finish_request(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 334, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python2.7/SocketServer.py", line 651, in __init__
    self.handle()
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 340, in handle
    self.handle_one_request()
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 328, in handle_one_request
    method()
  File "/.../env/local/lib/python2.7/site-packages/prometheus_client-0.0.7-py2.7.egg/prometheus_client/__init__.py", line 404, in do_GET
    self.wfile.write(generate_latest(REGISTRY))
  File ".../env/local/lib/python2.7/site-packages/prometheus_client-0.0.7-py2.7.egg/prometheus_client/__init__.py", line 392, in generate_latest
    for k, v in labels.items()]))
AttributeError: 'NoneType' object has no attribute 'replace'

There are several ways around this:

• Raise a ValueError in c.labels() if the label values are not all string-like
• In c.labels() or in generate_latest, coerce all label values to the unicode (or string) type
• In generate_latest, drop labels that can't be rendered, but render the other labels for that metric.
• In generate_latest, drop metrics that can't be rendered, but render the rest.

I'm not a fan of the options that drop data, because there's no worse monitoring than monitoring that lies to you. I think coercing to string may work, but I'd prefer if the code could raise. It already raises if the code provides the wrong number of label values, so raising on the wrong type of label values isn't unexpected.

[brian-brazil]

I think coercing is the best approach, as that's probably what the user intended.