New Server Session Monitor for SAU (Simultaneously Active Users), which will provide important informations for DAU (Active Daily Users) and MAU (Monthly Active Users).
The monitor is based on Meteor server sessions and will store the data in a new collection: rocketchat_sessions.

The approach of this implementation will deal with Connection events, such as onConnection and onClose, as well as dealing with Account events, such as onLogin and onLogout.

The SAU monitor will use the Meteor server sessions to update the session activities in DB. The monitor will ping the active sessions every minute, updating the lastActivityAt field on session documents.

The session lifecycle is stored per day, so when the server date is changed, the current sessions will be created on the new day. This behaviour is designed to facilitate the collector process, which will aggregate the session life cycle and store it in another collection of statistics.

The regular(non-mobile) session documents will look like the doc below:

{
	"_id" : "XSM2orqtSPWGYoQT2",
	"year" : 2018,
	"month" : 7,
	"day" : 20,
	"instanceId" : "LPvDxJh8fyDy6QsYz",
	"sessionId" : "ySTQpjSifhFfXWDis",
	"userId" : "uk294ojDzhYFQK53P",
	"ip" : "127.0.0.1",
	"host" : "localhost:3000",
	"browser" : {
		"name" : "Chrome",
		"version" : "67.0.3396.99",
		"major" : "67"
	},
	"os" : {
		"name" : "Mac OS",
		"version" : "10.13.6"
	},	
	"createdAt" : ISODate("2018-07-20T17:06:32.480Z"),  //when the session is created
	"lastActivityAt" : ISODate("2018-07-20T17:10:01.166Z"), //when the session monitor updates current sessions in bucket storage
	"loginAt" : ISODate("2018-07-20T17:06:32.491Z"), //user login
	"logoutAt" : ISODate("2018-07-20T17:10:07.311Z") //user logout
}

The mobile session documents will look like the doc below:

{
	"_id" : "YLoJ68zjA6WfNFt3b",
	"day" : 26,
	"instanceId" : "LPvDxJh8fyDy6QsYz",
	"month" : 7,
	"sessionId" : "wTrw5SNHa7wfudxaz",
	"year" : 2018,
	"ip" : "200.34.239.117,127.0.0.1",
	"host" : "d16877f6.ngrok.io",
	"os" : {
		"name" : "iOS",
		"version" : "11.4.1"
	},
	"device" : {
		"type" : "mobile"
	},
	"app" : {
		"name" : "RC Mobile",
		"version" : "v3.0.2",
		"bundle" : "(202)"
	},
	"createdAt" : ISODate("2018-07-26T18:57:49.475Z"),
	"userId" : "uk294ojDzhYFQK53P",
	"loginAt" : ISODate("2018-07-26T18:57:49.812Z"),
        "lastActivityAt" : ISODate("2018-07-26T18:58:03.834Z"),
	"closedAt" : ISODate("2018-07-26T18:58:03.834Z")
}

This new feature is being implemented in 3 steps:

• Model design (sessions)
• Monitor Implementation
• Tests

This PR will close the issue number: #11461.

Once the implementation is approved, we need to think about the creation of a cron job that will collect the data every day, aggregating the data and storing in two other collections: SAU and MAU.