[prometheus_check] check does not pickup summary metrics

[waller-wg2]

Summary metrics seems to get lost in parse_metric_family

Additional environment details (Operating System, Cloud provider, etc): Debian 9

Steps to reproduce the issue:

1. add any valid prometheus check
2. make sure at least one metric of the scraped target is of type summary
3. implement check accordning to https://docs.datadoghq.com/agent/prometheus/

Describe the results you received: Summary metric is missing

Describe the results you expected: Summary metric present

Additional information you deem important (e.g. issue happens only occasionally): Seems the parse_metric_family skips the summary cause the metric name of a summary is never part of messages since that holds the two actual metric names (x_count, x_sum) https://github.com/DataDog/dd-agent/blob/ac41f34c5edf5df91f09ac55bf7fb542f550fb99/checks/prometheus_mixins.py#L189

Needs similar handling as histogram?

[dnfehren]

Just ran into this as well. Is this due to the python client for prometheus? https://github.com/prometheus/client_python#summary says The Python client doesn't store or expose quantile information at this time.