public Map<String, MemberShape> getInitialMessageMembers() {
        return initialMembers;
    }
Gets the initial message members that form the initial-request
or initial-response of the event stream.

<p>The initial message members are the members of the input or
output of an operation that are not the target of an event stream.

<p>The returned map is a map of member names to
{@link MemberShape}s. Use {@link #getInitialMessageTargets} to get
a mapping of member names to the shapes targeted by a member.

<pre>{@code
EventStreamIndex index = EventStreamIndex.of(model);
EventStreamInfo info = index.getInputInfo(myShapeId);

for (MemberShape member : info.getInitialMessageMembers()) {
    System.out.println("Initial message member: " + member);
}
}</pre>

@return Returns the initial event members (if any).
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/EventStreamInfo.java