grpc-js: Allow garbage collection of IDLE channels

[murgatroid99]

This addresses #2893 to the greatest extent reasonable.

This PR includes 3 changes to allow garbage collection of IDLE channels:

• Modify the subchannel wrapper used by the channel so that it does not register itself until it has a ref, to handle the fact that pick_first may never ref some subchannels.
• Don't run the callRefTimer until a call is started, and stop it when the channel goes IDLE.
• Move the channelz info into a separate object, so that the reference from channelz only keeps that object alive instead of the whole channel.

A caveat here is that repeatedly creating and discarding channels without closing them will still leak some memory with channelz, because the channel is never unregistered with channelz, so channelz needs to retain the associated data. This does not happen if channelz is disabled for a channel.

[igorescobar-moonpay]

Do you think this issue could be related somehow? googleapis/nodejs-pubsub#2016

[karliky]

@igorescobar-moonpay are you properly closing the clients?

[murgatroid99]

@igorescobar-moonpay The purpose of this change was to minimize the memory impact of continuously creating and discarding client objects. However, the best strategy is to not do that at all. For any given service on a particular server, you should only create one client object, and use it for the lifetime of the process. If for whatever reason your only option is to keep creating new clients, you should always explicitly close them when you discard them.

The heap dump screenshots in the linked issue show that a large amount of retained memory is in the Client class, which implies that you are in fact creating many client objects.

[igorescobar-moonpay]

@igorescobar-moonpay The purpose of this change was to minimize the memory impact of continuously creating and discarding client objects. However, the best strategy is to not do that at all. For any given service on a particular server, you should only create one client object, and use it for the lifetime of the process. If for whatever reason your only option is to keep creating new clients, you should always explicitly close them when you discard them.

The heap dump screenshots in the linked issue show that a large amount of retained memory is in the Client class, which implies that you are in fact creating many client objects.

I appreciate you taking the time to look into the screenshots and giving us some pointers! I will have a look!

[igorescobar-moonpay]

Thanks a lot for the pointer @murgatroid99! The issue was fixed, more here: googleapis/nodejs-pubsub#2016 (comment)