push_to_gateway: handling of spaces in group key values

[jtougas]

Spaces in a group key value, come back as "+":

reproduction: (assumes docker interface at 172.17.0.1)

1. Start PGW: docker run -it --rm -p 9091:9091 prom/pushgateway:v1.6.2
2. Push metrics and read back

   docker run -it -e PIP_ROOT_USER_ACTION=ignore --rm python:3.10 bash -c "
   pip --disable-pip-version-check install -q prometheus-client && \
   python3 << 'EOF'
   from prometheus_client import push_to_gateway
   from prometheus_client.metrics_core import GaugeMetricFamily
   from prometheus_client.registry import Collector, CollectorRegistry   
   m1 = GaugeMetricFamily(name='spaces_in_group_key_values_status', documentation='N/A')   
   class _CustomCollector(Collector):
       def collect(self):
           yield m1   
   registry = CollectorRegistry()
   registry.register(_CustomCollector())   
   push_to_gateway('http://172.17.0.1:9091',job='job_name',grouping_key={'label': 'value with spaces'},registry=registry)
   EOF
   "; curl -s http://172.17.0.1:9091/metrics | grep spaces
   

expected output:

push_failure_time_seconds{instance="",job="job_name",label="value with spaces"} 0
push_time_seconds{instance="",job="job_name",label="value with spaces"} 1.7290223660476923e+09

actual output:

push_failure_time_seconds{instance="",job="job_name",label="value+with+spaces"} 0
push_time_seconds{instance="",job="job_name",label="value+with+spaces"} 1.7290223660476923e+09