Shard count handling improvements at scale

[bkuhl]

discord/discord-api-docs#387 indicates that when bots exceed 100k guilds the shard count must be divisible by 16. I'm currently quite occupied with some issues related to my bot and the intents rollout, but wanted to document this here as it would be a nice feature to see baked in. In Discord's developer support, they indicate the multiple of 16 is a requirement at 250k servers as part of the large bot sharding migration.

[jogramming]

There's no way for this library to auto detect that hah, the recommended shard count discord provides you im pretty sure is a multiple of 16 at that scale anyways.

[bkuhl]

Unfortunately Discord's recommendations didn't line up with it - which I found quite odd. However I was able to accomplish it with this:

	...
	if recommended < 2 {
		manager.SetNumShards(2)
	}

	// Larger bots see better performance in bots with a multiple of 16 (a requirement when it exceeds 250k guilds)
	manager.SetNumShards(int(math.Round(float64(recommended) / float64(16)) * 16))