Arrange Datadog Redis integrations more usefully (#14185)

This commit solves two problems:

1. Redis cache usage is tracked under `redis-rpush` because they use the
   same Redis URL and `redis-rpush` overrides `redis`.
2. Sidekiq jobs can enqueue an unbounded number of other Sidekiq jobs.
   The instrumentation cost alone of enqueuing hundreds of thousands of
   other Sidekiq jobs can be prohibitive, similar to DEBUG-level
   logging. This commit wraps it in a conditional so it must be enabled
   explicitly and can be turned on and off at will without a code
   change.

app/lib/forem_stats_drivers/datadog_driver.rb

@@ -19,12 +19,26 @@ module ForemStatsDrivers
         c.use :httpclient, split_by_domain: false
         c.use :httprb, split_by_domain: true
         c.use :rails
-        c.use :redis, service_name: "redis", describes: { url: ENV["REDIS_URL"] }
-        c.use :redis, service_name: "redis-rpush", describes: { url: ENV["REDIS_RPUSH_URL"] }
-        c.use :redis, service_name: "redis-sessions", describes: { url: ENV["REDIS_SESSIONS_URL"] }
-        c.use :redis, service_name: "redis-sidekiq", describes: { url: ENV["REDIS_SIDEKIQ_URL"] }
+
         c.use :rest_client
         c.use :sidekiq
+
+        # Multiple Redis integrations to split Redis usage per-instance to
+        # accommodate having a different Redis instance for each use case.
+
+        c.use :redis, service_name: "redis-rpush", describes: { url: ENV["REDIS_RPUSH_URL"] }
+        c.use :redis, service_name: "redis-sessions", describes: { url: ENV["REDIS_SESSIONS_URL"] }
+
+        # Sidekiq jobs that spin up thousands of other jobs end up consuming a
+        # *lot* of memory on instrumentation alone. This env var allows us to
+        # enable it only when needed.
+        if ENV["DD_ENABLE_REDIS_SIDEKIQ"] == "true"
+          c.use :redis, service_name: "redis-sidekiq", describes: { url: ENV["REDIS_SIDEKIQ_URL"] }
+        end
+
+        # Generic REDIS_URL comes last, allowing it to overwrite any of the
+        # above when multiple Redis use cases are backed by the same Redis URL.
+        c.use :redis, service_name: "redis", describes: { url: ENV["REDIS_URL"] }
       end
 
       Datadog::Statsd.new