fix: only keep track of sockets opened using domain name

[jackwotherspoon]

The list sockets on the MonitoredCache should only keep track of
sockets opened for connections using a domain name.

It should not keep track of all sockets for regular connections.

This is what it is currently doing:

cloud-sql-jdbc-socket-factory/core/src/main/java/com/google/cloud/sql/core/Connector.java

Line 141 in a5329ac

instance.addSocket(socket);

Sockets are only removed from the list if the connections were made using a domain name:

cloud-sql-jdbc-socket-factory/core/src/main/java/com/google/cloud/sql/core/MonitoredCache.java

Lines 92 to 96 in a5329ac

	synchronized (sockets) {
	for (Iterator<Socket> it = sockets.iterator(); it.hasNext(); ) {
	Socket socket = it.next();
	if (socket.isClosed()) {
	it.remove();

Thus for sockets not using a domain name and using the default instance connection name pattern, the sockets list will continue to grow and grow without being cleaned up.

This PR adds the proper check to gate adding to the sockets list for only connections using a domain name.

Fixes #2129

[jackwotherspoon]

@hessjcg do you mind adding a unit test to verify that the Connector using an instance connection name does not add to the socket list?

[jackwotherspoon]

LGTM, thanks for the tests! ✅