rfc: Allow reading current context for another thread

I've been working on integrating Datadog's continuous profiler with
opentelemetry traces (see DataDog/dd-trace-rb#1568).

The profiler runs on a background thread, and needs to be able to
access the current context for a thread, to be able to get the
current span's trace id and span ids (if any active).

To do so, I was originally using

```ruby
thread = ...
::OpenTelemetry::Trace.current_span(thread[::OpenTelemetry::Context::KEY])
```

Unfortunately, after open-telemetry#807, this interface was changed, and more
importantly, `Context::KEY` was removed and replaced with
`Context::STACK_KEY`. `STACK_KEY` was marked as `private_constant`.

With 1.0.0.rc2, the only way of getting this information is by
relying on private implementation details, which isn't great.

Thus, I would like to ask if it'd be reasonable to add an optional
`thread` parameter to `Context.current`. This would make it easy to
access the needed information, and it would even be more future-proof
as the outside code doesn't need to care anymore where and how
the context is stored.

api/lib/opentelemetry/context.rb

@@ -28,8 +28,9 @@ def create_key(name)
       # Returns current context, which is never nil
       #
       # @return [Context]
-      def current
-        stack.last || ROOT
+      def current(thread = Thread.current)
+        # Avoid calling #stack directly so as to not have races in initializing the storage
+        thread[STACK_KEY]&.last || ROOT
       end
 
       # Associates a Context with the caller's current Fiber. Every call to

api/test/opentelemetry/context_test.rb

@@ -333,5 +333,25 @@
         _(Context.current[bar_key]).must_be_nil
       end
     end
+
+    it 'allows accessing the current context for another thread' do
+      another_thread_context = Queue.new
+
+      another_thread = Thread.new do
+        ctx = Context.empty.set_value(foo_key, 'bar')
+        Context.with_current(ctx) do
+          another_thread_context << Context.current
+          sleep
+        end
+      end
+
+      ctx = another_thread_context.pop
+
+      _(Context.current(another_thread)).must_equal(ctx)
+      _(Context.current).wont_equal(ctx)
+
+      another_thread.kill
+      another_thread.join
+    end
   end
 end