Jaeger: Converge on JAEGER_SAMPLING_ENDPOINT env variable for client's sampling

[yurishkuro]

Update 2020-02-07

per discussion https://github.com/jaegertracing/jaeger/issues/1971#issuecomment-570631770, we settled on the name JAEGER_SAMPLING_ENDPOINT. I'm adding the tracking:

• [x] Go client https://github.com/jaegertracing/jaeger-client-go/pull/511
• [ ] Java client
• [ ] Node client
• [ ] Python client
• [x] C# client https://github.com/jaegertracing/jaeger-client-csharp/pull/165
• [x] C++ client https://github.com/jaegertracing/jaeger-client-cpp/pull/200
• [ ] Documentation matrix update

Original ticket

A bunch of tickets and PRs flying about, without a firm agreement on what we want to call this env variable.

Two candidates:

1. JAEGER_CONFIG_MANAGER_HOST_PORT https://github.com/jaegertracing/jaeger-client-java/pull/554/files#r221994309
2. JAEGER_CONFIG_ENDPOINT https://github.com/jaegertracing/jaeger-client-go/issues/282#issuecomment-537415291

Drawbacks:

1. JAEGER_CONFIG_MANAGER_HOST_PORT: Does not allow http vs. https, which may be necessary/useful in some situations
2. JAEGER_CONFIG_ENDPOINT: Consistent with JAEGER_ENDPOINT for submitting spans, but misleading, because JAEGER_ENDPOINT is the actual URL like http://jaeger-collector:14268/api/traces, but for configuration it's a _base url_ only, to be used with suffix endpoints like /sampling or /baggage

[yurishkuro]

cc @jpkrohling @pavolloffay

[jpkrohling]

I'm in favor of having a configuration option that accepts a URL, which contains all the components required to derive the actual endpoint value (protocol, host, port, base path).

The other obvious name for the property would be JAEGER_CONFIG_URL, but it has the same drawback as JAEGER_CONFIG_ENDPOINT, as it's not a complete URL (lacks the actual path). JAEGER_CONFIG_BASE_URL would be better in that sense, but I think it's unnecessarily verbose.

[pavolloffay]

JAEGER_CONFIG_ENDPOINT is fine. First I didn't consider http/https problem.

Do we expect people using different endpoints? If no we could require specifying just the root path and set the base in the implementation to make it easier for users.

[yurishkuro]

Conclusion: go with JAEGER_CONFIG_ENDPOINT, but make it explicit in the description that this is a "base URL".

Agreed?

[yurishkuro]

Update: per discussion https://github.com/jaegertracing/jaeger/issues/1971#issuecomment-570631770, we settled on the name JAEGER_SAMPLING_ENDPOINT

[Falco20019]

C# can be checked as done as it's merged and released as of 0.3.7.