Added feature to get password from Vault

[Dmitry-Eremeev]

To use the feature we need to set the following options in config section 'client':

use_vault = true vault_address = http://<IP_ADDRESS:<IP_PORT> vault_role_id = <ROLE_ID> vault_secret_id = <SECRET_ID> vault_secret_path = <deployments/<UNIT>/dev/user/passwords_yml> vault_secret_mount_path = <secret_v2> credential_name_in_vault_secret = <prometheus_mysql_exporter_database_password>

https://github.com/prometheus/mysqld_exporter/issues/883