Add Prometheus

[Janni2006]

This pull request introduces several updates and new features to the opencast-ca-display project, including Go module updates, Prometheus metrics integration, and improved error handling. The most important changes are summarized below:

Go Module Updates:

• Updated the Go version from 1.20 to 1.21 and added toolchain go1.24.0.
• Added new dependencies such as github.com/beorn7/perks, github.com/cespare/xxhash/v2, and several Prometheus-related packages.

Prometheus Metrics Integration:

• Introduced Prometheus metrics support by adding necessary imports and defining new structs and functions for metrics collection. [1] [2]
• Added a new Metrics section to the Config struct and updated the loadConfig function to handle metrics configuration. [1] [2]
• Implemented a new setupMetricsRouter function to handle Prometheus metrics endpoint.

Error Handling Improvements:

• Improved error handling in the setupRouter function by adding checks for potential errors and logging them appropriately.

Configuration File Update:

• Updated the opencast-ca-display.yml configuration file to include a new metrics section for enabling Prometheus metrics.