Redis 5 Instrumentation

[TonyCTHsu]

What does this PR do?

Instrument Redis 5

** Notable changes **

• Redis 5 extracts its client to another gem redis-client
• Public interface for instrumentation, see our changes
• Offer custom for each specific client
• redis commands are now strings, used to be symbols, see our changes

Before

customer_cache = Redis.new
invoice_cache = Redis.new

Datadog.configure_onto(customer_cache, service_name: 'customer-cache')
Datadog.configure_onto(invoice_cache, service_name: 'invoice-cache')

customer_cache.get(...)
invoice_cache.get(...)

After

customer_cache = Redis.new(custom: { datadog: { service_name: 'custom-cache' } })
invoice_cache = Redis.new(custom: { datadog: { service_name: 'invoice-cache' } })

customer_cache.get(...)
invoice_cache.get(...)

#2373