Missing `DD_TRACE_AGENT_URL` for go configurations

[13013SwagR]

Documentation Page

https://github.com/DataDog/documentation/tree/master/content/en/tracing/trace_collection/library_config/go.md

Issue Description

The documentation doesn't mention DD_TRACE_AGENT_URL which is pretty important to configure this
see the documentation in the code here https://github.com/DataDog/dd-trace-go/blob/main/internal/agent.go#L24-L32