add proxy feature for /pmapi URI

[fireboyvt]

Hosts to be analyzed is in IDC which can not be accessed from my laptop. so i cannot connect directly to Host to be analysed. But I can get accessed to a Nginx proxy which can set One IDC host as upstream and this host can access any IDC host.It will be vary convenient if vector can proxy /pmapi request to target IDC host which need to be analyzed.

[natoscott]

@fireboyvt Grafana and the grafana-pcp Vector datasource offer this functionality.

[fireboyvt]

3q，that works