Add GOOS/GOARCH to agent_build_info metric

[rfratto]

Adding more platform information in to agent_build_info metric would be useful for being able to determine which agents are running on which platforms.

[tpaschalis]

This was added to prometheus/common#403 so we already have this information available.

curl -s localhost:12345/metrics | grep agent_build_info
# HELP agent_build_info A metric with a constant '1' value labeled by version, revision, branch, goversion from which agent was built, and the goos and goarch for the build.
# TYPE agent_build_info gauge
agent_build_info{branch="",goarch="arm64",goos="darwin",goversion="go1.20",revision="230e1fe05531dfe890c2f72c738105e5581c940d-modified",tags="netgo",version=""} 1

Closing this issue.