Add a field to the metrics index to indicate if a user signed-up based on a referral from a Pro channel

This will help in tracing the activity of those users and check if they are active or not in order to reward the Pro channel

• Add a pro_referrer boolean to the metrics index
• Add the referrer guid (already present for signup)
• Wherever we write to the metrics index and have a referrer, write the referrer guid and the boolean if they are a referrer