Support service discovery in esctl configuration

[f9n]

Description

The host and port pair configurations can be retrieved from the service discovery tools (consul, etcd, zookeeper) or rest api.

Motivation

In dynamic environment, host and port pair can be changed. So this configuration definition will be stale.

Additional Context

• https://prometheus.io/docs/prometheus/latest/configuration/configuration/#http_sd_config
• https://prometheus.io/docs/prometheus/latest/configuration/configuration/#consul_sd_config