Fix Redis configuration pin

[TonyCTHsu]

What does this PR do?

Current redis instrumentation is looking for pin on client object instead of the redis instance object. Hence it is different from the behavior the document claimed.

require 'redis'
require 'ddtrace'

Datadog.configure do |c|
  c.tracing.instrument :redis # Enabling integration instrumentation is still required
end

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')

# Traced call will belong to `customer-cache` service
customer_cache.get(...)
# Traced call will belong to `invoice-cache` service
invoice_cache.get(...)

The PR implements patches provide mean to delegate the pin the be accessible to the client object.

[codecov-commenter]

Codecov Report

Merging #2363 (b298998) into master (af16512) will decrease coverage by 0.00%.
The diff coverage is 83.51%.

@@            Coverage Diff             @@
##           master    #2363      +/-   ##
==========================================
- Coverage   98.34%   98.33%   -0.01%     
==========================================
  Files        1102     1101       -1     
  Lines       58929    58938       +9     
==========================================
+ Hits        57954    57959       +5     
- Misses        975      979       +4     

Impacted Files	Coverage Δ
lib/datadog/core/configuration/settings.rb	97.51% <ø> (ø)
...filing/collectors/cpu_and_wall_time_worker_spec.rb	91.19% <71.42%> (-8.81%)	⬇️
spec/datadog/profiling/native_extension_spec.rb	96.80% <92.85%> (-0.70%)	⬇️
...g/profiling/collectors/cpu_and_wall_time_worker.rb	97.14% <100.00%> (ø)
...b/datadog/tracing/contrib/redis/instrumentation.rb	100.00% <100.00%> (ø)
lib/datadog/tracing/contrib/redis/patcher.rb	100.00% <100.00%> (ø)
spec/datadog/tracing/contrib/qless/support/job.rb	94.28% <100.00%> (ø)
spec/datadog/tracing/contrib/redis/miniapp_spec.rb	100.00% <100.00%> (+1.53%)	⬆️
spec/datadog/tracing/contrib/redis/redis_spec.rb	99.50% <100.00%> (ø)
lib/datadog/core/workers/polling.rb	96.55% <0.00%> (-3.45%)	⬇️
... and 1 more

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[zenspider]

should prolly be

super(options.merge(redis_instance: self))