Fix instrumentation for ibm https url connection connect

[laurit]

Instrumentation of IBM implementation of https url connection fails when connect is called before getInputStream/getOutputStream with

java.lang.IllegalStateException: Already connected
at sun.net.www.protocol.http.HttpURLConnection.setRequestProperty(HttpURLConnection.java:3214)
at io.opentelemetry.javaagent.instrumentation.httpurlconnection.RequestPropertySetter.set(RequestPropertySetter.java:16)
at io.opentelemetry.javaagent.instrumentation.httpurlconnection.RequestPropertySetter.set(RequestPropertySetter.java:11)
at io.opentelemetry.javaagent.shaded.io.opentelemetry.api.trace.propagation.W3CTraceContextPropagator.inject(W3CTraceContextPropagator.java:123)
at io.opentelemetry.javaagent.shaded.io.opentelemetry.context.propagation.MultiTextMapPropagator.inject(MultiTextMapPropagator.java:52)
at io.opentelemetry.javaagent.shaded.instrumentation.api.instrumenter.PropagatingToDownstreamInstrumenter.start(PropagatingToDownstreamInstrumenter.java:28)
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1545)
at com.ibm.net.ssl.www2.protocol.https.b.getInputStream(b.java:74)

because IBM implementation overrides the connect method and does not delegate to the jdk implementation. We don't instrument the connect in IBM class because it is not included in

opentelemetry-java-instrumentation/instrumentation/http-url-connection/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/httpurlconnection/HttpUrlConnectionInstrumentation.java

Line 33 in 5c10b1f

return nameStartsWith("java.net.")

So we end up attempting to start the span in getInputStream where headers can't be modified any more. This PR adds instrumentation for plainConnect method that is called from the connect in the IBM class.

[trask]

thanks for including a test 👍