Prometheus static targets mode support

[Vonng]

Scenario

Currently, postgres instance identity is managed by Consul Service Registration.
Which is implemented as local consul service configuration file in /etc/consul.d/svc-***.json
It is a distributed approach that allows instances themselves to 'report' their identity.
It do have some potential security issues such as wrong identity because of malicious or accidentally configured.

Some may prefer a centralized approach. E.g manage all identity in the meta-node.
Since we already have those information in inventory

Proposal

Take prometheus for example.
We can provide an alternative option to use static target identity file rather than service discovery.
Which may reduce the hazard of operating and reloading consul.

We can generate prometheus configuration with inventory and label all instance properly.
It also makes it much easier to integrate with other provisioning systems.

[Vonng]

Resolved by 162bb69 and ea5877e