add a usable instrumenter for better configuration

[zachdaniel]

With this you can define your own instrumenter and customize either the otp_app or the tracer when doing so:

defmodule MyApp.SpandexInstrumenter do
  use Spandex.Instrumenter, otp_app: :my_app

  # OR

  use Spandex.Instrumenter, tracer: MyApp.Tracer
end

Resolves #20

[coveralls]

Pull Request Test Coverage Report for Build 938fc0f0-0811-410a-b3e1-8f08697fcb5a

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.7%) to 70.93%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/spandex_phoenix/instrumenter.ex	0	6	0.0%
<!--	Total:	0	6	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
lib/spandex_phoenix/instrumenter.ex	4	0.0%
<!--	Total:	4		-->

Totals
Change from base Build 6e9817a9-354c-4575-97c1-00cd059036db:	-1.7%
Covered Lines:	61
Relevant Lines:	86

---

💛 - Coveralls

[zachdaniel]

Sounds good

[sourcelevel-bot[bot]]

SourceLevel has finished reviewing this Pull Request and has found:

• 13 possible new issues (including those that may have been commented here).

You can see more details about this review at https://app.sourcelevel.io/github/spandex-project/spandex_phoenix/pulls/21.

[bdubaut]

This change should probably also be reflected in the library's README