UnknownFormatError: Unsupported content-type provided: text

[szyn]

Hi, I get the following error when using Datadog with Prometheus integration.

prometheus (3.0.0)
------------------
    Instance ID: prometheus:99fffad4f445db41 [ERROR]
    Total Runs: 2
    Metric Samples: 0, Total: 0
    Events: 0, Total: 0
    Service Checks: 1, Total: 2
    Average Execution Time : 17ms
    Error: Unsupported content-type provided: text
    Traceback (most recent call last):
    File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/base/checks/base.py", line 366, in run
        self.check(copy.deepcopy(self.instances[0]))
    File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/base/checks/prometheus/base_check.py", line 108, in check
        ignore_unmapped=True
    File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/base/checks/prometheus/mixins.py", line 392, in process
        for metric in self.scrape_metrics(endpoint):
    File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/base/checks/prometheus/mixins.py", line 366, in scrape_metrics
        for metric in self.parse_metric_family(response):
    File "/opt/datadog-agent/embedded/lib/python2.7/site-packages/datadog_checks/base/checks/prometheus/mixins.py", line 235, in parse_metric_family
        response.headers['Content-Type']))
    UnknownFormatError: Unsupported content-type provided: text

The cause of the error is mimetype='text'.
https://github.com/epoch8/airflow-exporter/blob/master/prometheus_exporter.py#L125

As prometheus/client_python, we have better to use text/plain instead of text.

[elephantum]

@szyn thanks for reporting and fixing!

[szyn]

@elephantum Thank you, too!