Apache Connector: add a config property to choose an HttpClientBuilder

[Mahoney]

Currently both the 4.x & 5.x Apache Connector Providers hard code the creation of the HttpClientBuilder.

4.x connector (3.1 branch):

class ApacheConnector implements Connector {
    ApacheConnector(final Client client, final Configuration config) {
        final HttpClientBuilder clientBuilder = HttpClientBuilder.create();
        this.client = configuredBuilder.build();
    }
}

5.x connnector (4.0 branch):

class Apache5Connector implements Connector {
    Apache5Connector(final Client client, final Configuration config) {
        final HttpClientBuilder clientBuilder = HttpClientBuilder.create();
        this.client = configuredBuilder.build();
    }
}

It would be convenient to be able to choose a different implementation of HttpClientBuilder. For instance, opentelemetry-apache-httpclient extends HttpClientBuilder in order to override a protected method.

I propose a new property Apache5?ClientProperties.CLIENT_BUILDER. ApacheConnector would extract the object at that property, use it if it is an instance of HttpClientBuilder, and fall back to HttpClientBuilder.create() if it is absent / not an instance of HttpClientBuilder.

Happy to contribute a PR if it would be accepted in principle.

[jansupol]

Would ApacheHttpClientBuilderConfigurator be the class you seek? Just register your implementation to the client and you may configure the HttpClientBuilder there.

[jansupol]

Apache5HttpClientBuilderConfigurator for Apache Http Client 5.

[Mahoney]

By that stage all the configuration of the HttpClientBuilder has been done, so while it is possible to return a different instance of a subtype of HttpClientBuilder from an ApacheHttpClientBuilderConfigurator, the implementation has to copy all the previous configuration onto it, which seems arduous and error prone.

[jansupol]

I see. There are multiple possibilities how to implement this.

If a property, I assume it would better be a supplier of the HttpClientBuilder.
Other option is to extend the Apache5HttpClientBuilderConfigurator by a create() method that would be invoked the first before configuring the builder.

Not sure which one is better at the moment. The latter seems more consistent, but would cause issues when multiple registered. They could be sorted by Priority, though. The first brings another property, but for a supplier this time. Not as type-safe as the interface option.