Relevent telegraf.conf

[[inputs.prometheus]]
  urls = ["http://localhost:8080/metrics"]

System info

Telegraf 1.20.4 MacOS 11.6.1

Docker

No response

Steps to reproduce

The issue is related to the Accept header of the Prometheus input plugin:

1. Let's say we have a service that prefers the OpenMetrics format or exclusively supports it
2. Since Telegraf accepts any media types (see: Accept any media type in the prometheus input #6745) it should be able to parse the OpenMetrics format in the response (because of */*)
3. But Telegraf is not able to parse the OpenMetrics format (see: Support for OpenMetrics standard #9547) so it fails with:

2021-12-09T02:43:40Z E! [inputs.prometheus] Error in plugin: error reading metrics for http://localhost:8080/metrics: reading text format failed: text format parsing error in line 118: invalid escape sequence '\"'

Expected behavior

Telegraf should only accept the formats it supports (no */*):

application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3

Actual behavior

Telegraf should not accept everything (see */*):

application/vnd.google.protobuf;proto=io.prometheus.client.MetricFamily;encoding=delimited;q=0.7,text/plain;version=0.0.4;q=0.3,*/*;q=0.1

In this case, if the service only support OpenMetrics, Telegraf should get 406 Not Acceptable instead of a response in the OpenMetrics format that it tries and fails to parse.

Additional info

Prometheus itself also defines */* as a fallback but it supports both formats (and it prefers OpenMetrics) so it does not get into any trouble if the client prefers or only supports OpenMetrics:

application/openmetrics-text; version=0.0.1,text/plain;version=0.0.4;q=0.5,*/*;q=0.1