Absinthe telemetry integration

[garthk]

In preparation for absinthe-graphql/absinthe#663: provide an easy means to hook up via :telemetry rather than Absinthe.Pipeline without the user having to write their own code to call [:telemetry.attach_many/4].

[MatteoJoliveau]

Hello @garthk! The absinthe PR has been merged and released in Absinthe 1.5. Do you think an update to opencensus_absinthe could be released with the integration with Telemetry? We're using this library to instrument Elixir microservices and this feature would greatly improve our code.