use router dispatch start events to update name of span

[garthk]

• fix compile
• fix plug dependency
• fix tests

[garthk]

The test is failing because your handle_route/4 is failing because the span context is :undefined because your handle_start/4 wasn't called because nobody sent a [:plug_adapter, :call, :start] because there's no adapter present.

[garthk]

Ok, I'm going to need to loop around and submit a PR for Plug.Test because it doesn't call into telemetry.execute/3 as required by [reads notes] Plug.Conn.Adapter.

[garthk]

I can make the test pass by calling :telemetry.execute/4 before MyRouter.call/2, but any users who don't run around doing the same to their tests will get a noisy error as our telemetry handler disconnects:

12:41:18.908 [error] Handler {OpentelemetryPlug, :plug_tracer_router_start} has failed and has been detached. Class=:error
Reason=:function_clause
Stacktrace = [
  {:ot_span_ets, :update_name, [:undefined, "/hello/:foo/*glob"],
   [file: '$HOME/opentelemetry_plug/deps/opentelemetry/src/ot_span_ets.erl', line: 131]},
  {:telemetry, :"-execute/3-fun-0-", 4,
   [file: '$HOME/opentelemetry_plug/deps/telemetry/src/telemetry.erl', line: 123]},
  {:lists, :foreach, 2, [file: 'lists.erl', line: 1338]},
  {MyRouter, :dispatch, 2, [file: 'lib/plug/router.ex', line: 277]},
  {MyRouter, :plug_builder_call, 2, [file: 'test/opentelemetry_plug_test.exs', line: 39]},
  {OpentelemetryPlugTest, :"test creates span", 1,
   [file: 'test/opentelemetry_plug_test.exs', line: 9]},
  {ExUnit.Runner, :exec_test, 1, [file: 'lib/ex_unit/runner.ex', line: 355]},
  {:timer, :tc, 1, [file: 'timer.erl', line: 166]},
  {ExUnit.Runner, :"-spawn_test_monitor/4-fun-1-", 4, [file: 'lib/ex_unit/runner.ex', line: 306]}
]

We can compensate for that, having handle_route/4 call handle_start/4 if the context is :undefined, but there's no similarly easy way to get handle_stop/4 called with a finished conn, which brings us back to trying to write advice for our users on how to write a test that'll prove their OT integration works. What do you reckon? This getting messy enough you'd ponder switching back to @behaviour Plug for integration?

[garthk]

Reckon I'll apply José's hint from elixir-plug/plug#945 and take a test-only dependency on plug_cowboy, and have handle_route and handle_stop no-op if the span context is :undefined.

[garthk]

Incorporates #1 for one-click merge convenience.

[seancribbs]

You could use Enum.flat_map/2. Is the concern efficiency?

[garthk]

Kinda doubt it; 𝑛=2. Just another tell that the author comes more from the Erlang side of things? I'd flat map with empty lists instead of false, yeah: this part of the diff is just mix format doing its thing while I edit elsewhere.