CBG-3637: Stat definitions use a named object for each stat definition

[IsaacLambat]

CBG-3637

Each stat has its own named object instead of having the name as a field in the object. Labels are also now ordered.

Example:

	"sgw_replication_sgr_deltas_sent": {
		"labels": [
			"database",
			"replication"
		],
		"help": "The total number of deltas sent.",
		"added": "3.0.0",
		"stability": "committed",
		"type": "counter"
	},
	"sgw_replication_sgr_docs_checked_recv": {
		"labels": [
			"database",
			"replication"
		],
		"help": "The total number of document changes received over changes message.",
		"added": "3.0.0",
		"stability": "committed",
		"type": "counter"
	},
	"sgw_replication_sgr_docs_checked_sent": {
		"labels": [
			"database",
			"replication"
		],
		"help": "The total number of documents checked for changes since replication started. This represents the number of potential change notifications pushed by Sync Gateway. This is not necessarily the number of documents pushed, as a given target might already have the change and this is used by Inter-Sync Gateway and SG Replicate. This metric can be useful when analyzing replication history, and to filter by active replications.",
		"added": "3.0.0",
		"stability": "committed",
		"type": "counter"
	},
	"sgw_replication_sgr_num_attachment_bytes_pulled": {
		"unit": "bytes",
		"labels": [
			"database",
			"replication"
		],
		"help": "The total number of bytes in all the attachments that were pulled since replication started.",
		"added": "3.0.0",
		"stability": "committed",
		"type": "counter"
	},

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api