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.