Add POST support to exporter

[RDruon]

Add POST support to the exporter.

This patch is based on a fork of https://github.com/MindFlavor/prometheus_exporter_base to enable POST supports. A PR has been opened to include this change in the official crate MindFlavor/prometheus_exporter_base#30

In the meantime, I can transfer the fork to the whamcloud org.

With patch

[root@node1 ~]# curl -X POST -I http://localhost:32221/metrics
HTTP/1.1 200 OK
content-type: text/plain; version=0.0.4
content-length: 4766
date: Mon, 26 Feb 2024 11:34:08 GMT
[root@node1 ~]# curl -X GET -I http://localhost:32221/metrics
HTTP/1.1 200 OK
content-type: text/plain; version=0.0.4
content-length: 4766
date: Mon, 26 Feb 2024 11:34:12 GMT

Without patch (405 Method Not Allowed):

[root@node1 ~]# curl -X POST -I http://localhost:32221/metrics
HTTP/1.1 405 Method Not Allowed
content-length: 0
date: Mon, 26 Feb 2024 11:27:12 GMT

[root@node1 ~]# curl -X GET -I http://localhost:32221/metrics
HTTP/1.1 200 OK
content-type: text/plain; version=0.0.4
content-length: 4766
date: Mon, 26 Feb 2024 11:27:18 GMT

[jgrund]

There is no description of why this change is needed.

[RDruon]

A software (that can't be named here) retrieve metrics through "POST" instead of "GET".
The software has been updated to support both "POST" and "GET", closing.