grpc: do not use balancer attributes during address comparison

[easwars]

Summary of changes:

• Make of copy of received addresses in the clientconn code during subchannel creation and address update operation
  • clear out the balancer attributes field in this address list, thereby ensuring that changes in balancer attributes don't lead to connection churn
• Minor improvements to rendering of attributes in logs
• Make it possible to pass a net.Listener to the stubserver
  • this allows tests to use a wrapped listener that notifies the test upon connection establishment

Fixes internal issue: b/290272899

RELEASE NOTES:

• grpc: eliminate connection churn during an address update that differs only in balancer attributes

[dfawley]

Sad that string doesn't implement Stringer 😞

To consolidate these code paths, and to avoid printing literally ", " if key & val are both not string-y, maybe we should have something like this?:

func str(x interface{})  string {
	if v, ok := x.(fmt.Stringer); ok {
		return v.String()
	} else if v, ok := x.(string); ok {
		return v
	}
	return fmt.Sprintf("<%p>", x)
}

[easwars]

Done. Had to add a "%q" format string to ensure that the key and value strings are quoted.

[dfawley]

This is not a parameter for Listen or Dial. Can you document this please and indicate Network & Address aren't used if this is set, and that Target is the responsibility of the user to set properly when Listener is used?

[easwars]

My code search reveals that nobody is setting Network, Address or Target.

Address is used as a read-only field by numerous tests to retrieve the address of the stub server. Should we just get rid of Network and Target and make Address as a read-only field, or make it a getter method? (Can do in a follow up PR)

[dfawley]

SGTM!

[easwars]

Will do the cleanup in a follow up PR. Thanks.

[dfawley]

This one doesn't print literal quotes.

Why not keep it the way it was? Have this return the actual string and quote it when it's used?

[easwars]

Yeah, makes sense.