proposal: allow configuration of Trace Agent URL with DD_TRACE_AGENT_URL

[PonzaMatteo]

Problem Statement

Other SDKs (like the js and python one) allow to configure the trace agent URL using the env variable DD_TRACE_AGENT_URL while the Go SDK only allows using the combination of DD_AGENT_HOST + DD_TRACE_AGENT_PORT, this results in the following issues for our needs:

• the hardcoded protocol is HTTP, but we would need to utilize HTTPS
• it's not possible to omit the port in the agent URL since if left empty it defaults to 8126

Proposed Solution

Allow using DD_TRACE_AGENT_URL to override the combination of the other two variables.

[mcculls]

Note: another benefit of DD_TRACE_AGENT_URL is that you can use it to configure a UDS connection to the agent (ie. unix://<path> like in the other tracers)

[hush-hush]

This would also solve the issue of customer not being able to configure both a DogStatsD client and the tracer on the same host when using UDS:

ref: DataDog/datadog-go#201

[knusbaum]

I think DD_TRACE_AGENT_URL would be a more useful and consistent option than the current host + port combination for all the reasons mentioned here.

I think in addition to introducing DD_TRACE_AGENT_URL and having it override the other existing environment variables, we should deprecate the old ones and discourage their use.

[adawalli]

The fact that this is hard-coded goes against the defaults that are now used in the helm chart for agent deploys: https://github.com/DataDog/helm-charts/blob/main/charts/datadog/values.yaml#L355

8126 is not even exposed by default anymore - the Unix socket is used!