Scrape via Prometheus annotation

[applejag]

We're using this at our company. And we wanted to simplify it a bit to not host node-exporter in each pod. Thought we'd might as well publish our changes as a PR. :)

• Added YAML manifests to use in-cluster prometheus scraping

• Updated script

  • Writes to /shared_vol/scrape.txt instead
  • Buffers metrics in memory before writing to file, which resolves Fix "period of no metrics" in monitor-pv exporter #5
  • Use numfmt instead of hand-made function for parsing data format

• Removed old node-exporter-based YAML manifests

• Updated README.md