Add feature to use uid only without cid

[puttpotsawee]

Hi, I have recently come across GA lately and found your lib really useful.

I mostly use it to send some analytics data from the backend. My project analyzes mainly with UserId since we focus on user conversion etc.

Yesterday, I began to create custom dimensions for the user scope. I have an API fire the event when a user updates his status. I found the value in the custom dimensions are wrong when the user rapidly submits the update, even if it updates a different index of the custom dimensions.

2018-12-25T08:12:42.530Z universal-analytics Enqueued event ({ ea: 'Create Profile', ec: 'Backend', cd2: 'buyer', v: '1', tid: 'UA-XXXXXXXX', cid: '2aa7e81f-7787-46a1-a3fb-7d7335ba5127', t: 'event', uid: '5c21e678f001ab001a9200be' })

2018-12-25T08:12:42.541Z universal-analytics Enqueued event ({ ea: 'Update Profile', ec: 'Backend', cd1: 'in-house', cd2: 'seller', cd3: 'partner1', v: '1', tid: 'UA-XXXXXXXX', cid: '589f3a49-52cd-4fca-a7ad-c94a6dbb2b99', t: 'event', uid: '5c21e678f001ab001a9200be' })

My Custom dimensions are

Index	Field Name
cd1	subType
cd2	userType
cd3	userPartner

In the analytic dashboard, the UserExplorer shows this userID with 2 events, Create Profile and Update Profile. But these events appear to be in different sessions.

The custom dimensions are showing

{subType: undefined, userType: "buyer", userPartner: undefined}

I figure the cid is the problem for my case, I tried the same event firing directly to the Measurement Protocol API without the cid and the data works fine. In this case, it shows:

{subType: "in-house", userType: "seller", userPartner: "partner1"}

According to the Measurement Protocol Reference, the cid and uid can be either one.

So I create an option identifyByUserId which check if the uid is supplied then the auto-generate cid will not be run.