Prometheus plugin: support passing bearer_token directly

[quantonganh]

Feature Request

Opening a feature request kicks off a discussion.

Proposal:

Current behavior:

https://github.com/influxdata/telegraf/tree/master/plugins/inputs/prometheus

only support to pass bearer token as a path to file:

## Use bearer token for authorization
  # bearer_token = /path/to/bearer/token

Desired behavior:

It should be supported to pass token directly and rename bearer_token to bearer_token_file. Then it will be consistent with https://prometheus.io/docs/prometheus/latest/configuration/configuration/

Use case: [Why is this important (helps with prioritizing requests)]

We are using it for Grafana metrics. After generating the token, no need to copy it to a file.

[danielnelson]

Let's do the prometheus parser #4414 and then this can be done by setting headers in the http input.