Provide OpenTelemetry sink

[tomkerkhove]

Proposal

Provide OpenTelemetry sink so that people can use any exporter that they want to use.

Component

Scraper

Checklist

• [x] Automatically push scraped metrics
• [ ] Automatically push system metrics to scraper agent (https://github.com/tomkerkhove/promitor/pull/2096)
• [ ] Automatically push system metrics to resource discover agent
• [x] Use OpenTelemetry Collector locally
• [x] Update changelog
• [x] Update validation
• [x] Update startup logs
• [x] Update docs (https://github.com/promitor/docs/pull/28)
• [ ] Update Helm Chart (https://github.com/promitor/charts/runs/7667606177?check_suite_focus=true)
• [x] Update promitor.io (https://github.com/promitor/promitor.io/pull/22)
• [x] Update concept docs (https://github.com/promitor/promitor.io/pull/22)
• [x] Update OpenAPI specs
• [ ] Add system metrics to changelog