prometheus: add extra_args support for custom prometheus command-line flags

[djooberlee]

This pull request enhances the flexibility and maintainability of the Prometheus SaltStack configuration. The main improvements are the addition of support for extra Prometheus command-line arguments, the restoration of the retention time setting, and consistent Jinja2 formatting for better readability and maintainability.

Prometheus container enhancements:

• Added support for an extra_args field in the Prometheus instance configuration, allowing arbitrary command-line flags to be appended to the Prometheus container command (e.g., enabling remote write receiver) (prometheus/pillar.example, prometheus/prometheus.sls). [1] [2]

Jinja2 template consistency and maintainability:

• Standardized Jinja2 control structure formatting throughout prometheus/prometheus.sls for improved readability and to prevent whitespace-related bugs (e.g., {% ... %} changed to {%- ... %} in many places). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]