Set User-Agent in HTTP requests when not already set

Changelog for 0.27.9:

• PR #1843 Set default user agent in proxy client by @welteki

Commits
4e80b96 Set default user agent in proxy client by @welteki
1379805 Switch to Default HTTP client for Prometheus queries by @alexellis
546bfee Add User-Agent to Prometheus Queries by @alexellis
637b0b0 Updates to GitHub Actions versions by @alexellis
32b4117 Dockerfile linting and alpine linux update by @alexellis

Changes: 0.27.8...0.27.9

Generated by Derek