Add dalli support

[p-lambert]

Overview

Dalli integration will trace all calls to your memcached server:

require 'dalli'
require 'ddtrace'

# patch Dalli so all clients are traced
Datadog::Monkey.patch_module(:dalli)

client = Dalli::Client.new('localhost:11211', options)
client.set('abc', 123)

# to change Dalli service name, use the Pin class
pin = Datadog::Pin.get_from(client)
pin.service = 'primary-cache'

[palazzem]

@p-lambert let's write a bit of documentation for this contrib module.

[palazzem]

don't we need to patch / patch all something?

[palazzem]

will add that as a general example; unfortunately for Rails it will be:

require 'ddtrace'

# Tracing configuration
Rails.configuration.datadog_trace = {
  auto_instrument: true,
  auto_instrument_redis: true,
  auto_instrument_grape: true,
  default_service: ENV['RAILS_SERVICE'] || 'rails-local',
  default_database_service: ENV['RAILS_MYSQL_SERVICE'] || 'db-local',
  default_cache_service: ENV['RAILS_CACHE_SERVICE'] || 'cache-local',
  default_controller_service: ENV['RAILS_CONTROLLER_SERVICE'] || 'controller-local',
  trace_agent_hostname: ENV['DATADOG_TRACER'] || 'datadog'
}

Datadog::Monkey.patch_module(:dalli)
pin = Datadog::Pin.get_from(::Dalli)
pin.service = 'dalli-cache'

mostly because we have some things that are patched via Rails settings, and others that are not. I'd say to not add Dalli to the auto_instrument_* since we're planning to remove this settings in favor of a better way to configure our integrations.

[palazzem]

Let's wait for the CI that is green and merge it. I've updated the docs so the patch_module call is here, and we show also how to update the service name.

After the Configuration change, I think it will be far better. For now, let's stick to the legacy API.