OTLP export support

[Arend van Beelen jr.]

As part of this task, I will be adding support for exporting to the OTel Collector using OTLP.

Since we already bundle two methods for exporting (the Prometheus exporter and the push gateway), and we decided we no longer want to auto-start a webserver under the default configuration, I will rework how the exporter configuration works and will split the exporters into separate packages.

• Create package for the existing Prometheus exporter
• Create package for the existing push gateway exporter
• Create package for the new OTLP push exporter
• Update the core package and the init() function to work with these exporter packages.