Addes TLS support for exporter endpoint using prometheus exporter-toolkit library

[dr1nf3rn0]

Description

• Allows to configure pgpool2 prometheus exporter to use HTTPS endpoint

• Allows to use basic authentication for pgpool2 prometheus exporter endpoint

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Motivation and Context

• Current implementation lacks TLS support
• Inspired by prometheus node_exporter TLS implementation https://github.com/prometheus/node_exporter
• Prometheus Exporter Toolkit allows to share standard prometheus web configuration file format

Changes

• Added official Prometheus Exporter Toolkit as dependency https://github.com/prometheus/exporter-toolkit

• Added --web.config.file flag to exporter command line parameters. It specifies path to configuration file that can enable TLS or authentication. Configuration file conforms standard prometheus web configuration yaml file format

• Updated REAMDE.MD flags and usage.

Other

• Web config file configuration format specification: https://github.com/prometheus/exporter-toolkit/blob/master/docs/web-configuration.md

• Sample web-config.yml file format:

  # TLS and basic authentication configuration example.
  #
  # Additionally, a certificate and a key file are needed.
  tls_server_config:
      cert_file: server.crt
      key_file: server.key
  
  # Usernames and passwords required to connect.
  # Passwords are hashed with bcrypt: https://github.com/prometheus/exporter-toolkit/blob/master/docs/web-configuration.md#about-bcrypt.
  basic_auth_users:
      alice: $2y$10$mDwo.lAisC94iLAyP81MCesa29IzH37oigHC/42V2pdJlUprsJPze
      bob: $2y$10$hLqFl9jSjoAAy95Z/zw8Ye8wkdMBM8c5Bn1ptYqP/AXyV0.oy0S8m