Update OpenMetrics Content Type to 1.0.0 (prometheus#997)

OpenMetrics has been released and we support 1.0.0 not 0.0.1.

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

Author: csmarchbanks

prometheus_client/openmetrics/exposition.py

@@ -3,7 +3,7 @@
 
 from ..utils import floatToGoString
 
-CONTENT_TYPE_LATEST = 'application/openmetrics-text; version=0.0.1; charset=utf-8'
+CONTENT_TYPE_LATEST = 'application/openmetrics-text; version=1.0.0; charset=utf-8'
 """Content type of the latest OpenMetrics text format"""