Hide some tracing instrumentations behind environment variables

This commit disables Redis and ActiveSupport instrumentations. They can
still be enabeld when enable some specific environment variables.

Redis one captures all calls to Redis server. As a
typical web request results in numerous Redis calls, this
instrumentation generates more noises. This type of information is
arguably useful. Besides, folks can use the performance bar as a
replacement if they really want to have a deep look to Redis calls.

ActiveSupport instrumentation captures cache-related operations,
including read, write, fetch, delete, to name a few. We use Redis for
caching. It means each operation is duplicated twice. Furthermore, I
don't think cache operations offer any benefits.

Changelog: removed