[speedtest-prometheus] - adding speedtest-prometheus chart

[billimek]

Special notes for your reviewer:

Initial commit

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] Chart Version bumped
• [x] Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [radarr])