ProxyAuthenticator Support for OtlpHttp{Signal}Exporters

[jigarkb]

Is your feature request related to a problem? Please describe.
OtlpHttp{Signal}Exporters do support Proxy configuration. Thanks to this issue #6204. However, we don't expose the ProxyAuthenticator that OkHttp requires if want to use an authenticated proxy. See square/okhttp#3607 for more details on why we can't just set the Proxy-Authorization header.

Describe the solution you'd like
Enhance ProxyOptions class to support providing ProxyAuthenticator in addition to ProxySelector.

[jack-berg]

We have an experimental configuration optional called Authenticator modeled after the OkHttp Authenticator concept.

Its a bit hard to use right now.. Something like:

    OtlpHttpMetricExporterBuilder exporterBuilder =
        OtlpHttpMetricExporter.builder()
            .setEndpoint(...)
            .setProxyOptions(...);
    Authenticator.setAuthenticatorOnDelegate(
        exporterBuilder,
        () -> Collections.singletonMap("Proxy-Authorization", proxyAuthorizationToken()));
    OtlpHttpMetricExporter exporter = exporterBuilder.build();

See OkHttpHttpSender for details on how its translated to the Okhttp Authenticator.

Is there something specific about proxy authentication that makes our Authenticator insufficient?

[jigarkb]

@jack-berg The issue is okhttp won't accept Proxy-Authorization from normal Authenticator. It requires us to set proxyAuthenticator from builder. See https://github.com/square/okhttp/blob/90d2c07566ba552c5320b00249f61a9469270117/okhttp/src/main/kotlin/okhttp3/OkHttpClient.kt#L777-L786
and https://github.com/square/okhttp/blob/90d2c07566ba552c5320b00249f61a9469270117/okhttp/src/main/kotlin/okhttp3/OkHttpClient.kt#L869-L882

[jack-berg]

I see. I think it probably makes sense to add support for setting an Authenticator specifically for use with the proxy.

But really we ought to revisit Authenticator, which has been around for a while, yet doesn't feel particularly close to stabilizing, since its not supported in JdkHttpSender, or OkHttpGrpcSender, UpstreamGrpcSender. The concept seems sort of half-baked, and its API and semantics need to be better articulated so it can be supported outside of just OkHttp.

[jigarkb]

I realize the Authenticator implementation is still incomplete. In the meantime, would it be acceptable to proceed with adding the Authenticator to ProxyOptions and using it within OkHttpHttpSender? If so, I can contribute and open a PR for review.

[jack-berg]

Authenticator was removed in #6984, but I think it makes sense to add proxy overloads that allow additional headers to be provided. Something like:

OtlpHttpSpanExporter.builder()
  .setProxy(
     ProxyOptions.create(InetSocketAddress.createUnresolved("host", "port")),
     () -> Map.of("Proxy-Authorization", "...."))

I took a look at the source code and we would be able to incorporate this type of pattern into JdkHttpSender and OkHttpSender.

If anyone does want to work on this, you'll have to wait on #7151 to avoid conflicts.