Shut down the OTel SDK between collector-mode specs

The collector-mode shared context boots a full OTel SDK per example
via `start_agent` and never shut it down, leaking the batch span,
periodic metric reader and batch log processor threads (~460 live
threads by the end of a -collector CI run). The batch span thread was
doubly stranded: the context swaps in an in-memory tracer provider for
assertions, orphaning the booted one.

Shut the booted tracer provider down before the swap, and shut the
meter and logger providers down in an after hook. The after hook calls
`Appsignal::OpenTelemetry.shutdown` directly rather than
`Appsignal.stop`: stop's `Extension.stop` takes ~2 seconds per call,
which across every collector-mode example adds minutes to the suite.

[skip changeset]

Author: unflxw

spec/support/shared_contexts/collector_mode.rb

@@ -19,12 +19,26 @@
 
   before do
     start_agent(:options => { :collector_endpoint => "http://127.0.0.1:9090" })
+    # `Appsignal.start` booted a full SDK tracer provider backed by a
+    # BatchSpanProcessor (a background export thread). Shut it down before
+    # swapping in the threadless in-memory provider: after the swap it is
+    # unreachable and its thread would leak across examples.
+    ::OpenTelemetry.tracer_provider.shutdown
     ::OpenTelemetry.tracer_provider = tracer_provider
   end
 
-  # `clear_current_transaction!` in spec_helper clears the thread-local but
-  # not the attached OTel context. `complete_current!` does both.
-  after { Appsignal::Transaction.complete_current! }
+  after do
+    # `clear_current_transaction!` in spec_helper clears the thread-local but
+    # not the attached OTel context. `complete_current!` does both.
+    Appsignal::Transaction.complete_current!
+    # Shut the OTel SDK down so the meter and logger providers' background
+    # threads don't accumulate across the suite. The targeted shutdown, not
+    # `Appsignal.stop`: stop's `Extension.stop` takes ~2 seconds per call,
+    # which across every collector-mode example adds minutes to the suite.
+    # Runs before the global `Appsignal::OpenTelemetry.reset!` hook, so the
+    # `started?` gate inside the shutdown still passes.
+    Appsignal::OpenTelemetry.shutdown
+  end
 
   def root_span
     span_exporter.finished_spans.find { |s| [:server, :consumer].include?(s.kind) }