[FEATURE]: Display real-time viewer stats

[dgurns]

Is your feature request related to a problem? Please describe.
When watching a show, you want to see how many other people are watching too.

Describe the solution you'd like
A scalable way to fetch current realtime viewers on a given user page.

Describe alternatives you've considered
A few possible ways to do this, and probably others I haven't thought of yet:

• Utilize the Redis subscriptions package (graphql-redis-subscriptions) we're using for GraphQL subscriptions, since it keeps track of how many users are currently connected to a given artist page.
• Use Google Analytics and query their realtime API

Additional context

• This might be what we need to query: https://redis.io/commands/pubsub#pubsub-channels-pattern

Update:

• Can probably use Redis sorted sets. https://redis.io/commands/zadd
  • Client app sends a ping every minute with the current user ID and urlSlug they're on.
  • Add to Redis sorted set saved under the route like urlSlug
  • When adding, delete all keys over a minute old.
  • Get count for sorted set - this will represent all pings within the last minute.
  • Can use that as an estimate for the number of users currently on page.
  • Can probably do this within the same request/response: Send request with userId and urlSlug, get response with number of current viewers