Only attempt to record an exception if there is an active span. (#37)

There is an edge case, if you use `forward/4` and use Plug.ErrorHandler,
then when an exception reaches the outer router, then Plug.send_resp
will be called, triggering `[:phoenix, :endpoint, :stop]`, and the span
will be gone by the time the outer router gets the exception. This
causes this telemetry handler to crash and be detached.

Sequence of events:
- [:phoenix, :endpoint, :start]
- [:phoenix, :router_dispatch, :exception] (inner router)
- [:phoenix, :endpoint, :stop]
- [:phoenix, :router_dispatch, :exception] (outer router) ** here there is no span, crashes

instrumentation/opentelemetry_phoenix/lib/opentelemetry_phoenix.ex

@@ -164,6 +164,7 @@ defmodule OpentelemetryPhoenix do
         %{kind: kind, reason: reason, stacktrace: stacktrace} = meta,
         _config
       ) do
+    if OpenTelemetry.Span.is_recording(OpenTelemetry.Tracer.current_span_ctx()) do
       ctx = OpentelemetryTelemetry.set_current_telemetry_span(@tracer_id, meta)
 
       {[reason: reason], attrs} =
@@ -180,6 +181,7 @@ defmodule OpentelemetryPhoenix do
       # do not close the span as endpoint stop will still be called with
       # more info, including the status code, which is nil at this stage
     end
+  end
 
   defp http_flavor({_adapter_name, meta}) do
     case Map.get(meta, :version) do

instrumentation/opentelemetry_phoenix/test/opentelemetry_phoenix_test.exs

@@ -167,6 +167,44 @@ defmodule OpentelemetryPhoenixTest do
              Map.keys(:otel_attributes.map(event_attributes))
   end
 
+  test "records exceptions for nested Phoenix routers" do
+    OpentelemetryPhoenix.setup()
+
+    :telemetry.execute(
+      [:phoenix, :endpoint, :start],
+      %{system_time: System.system_time()},
+      Meta.endpoint_start(:exception)
+    )
+
+    :telemetry.execute(
+      [:phoenix, :router_dispatch, :start],
+      %{system_time: System.system_time()},
+      Meta.router_dispatch_start(:exception)
+    )
+
+    :telemetry.execute(
+      [:phoenix, :router_dispatch, :exception],
+      %{duration: 222},
+      Meta.router_dispatch_exception(:normal)
+    )
+
+    :telemetry.execute(
+      [:phoenix, :endpoint, :stop],
+      %{duration: 444},
+      Meta.endpoint_stop(:exception)
+    )
+
+    :telemetry.execute(
+      [:phoenix, :router_dispatch, :exception],
+      %{duration: 222},
+      Meta.router_dispatch_exception(:normal)
+    )
+
+    assert_receive {:span, _}
+
+    assert [_ | _] = :telemetry.list_handlers([:phoenix, :router_dispatch, :exception])
+  end
+
   test "records exceptions for Phoenix web requests with plug wrappers" do
     OpentelemetryPhoenix.setup()