fix: Replace Context stack on clear (#1598)

* fix: Replace Context stack on clear

* Document clear method

Author: fbogsany

api/lib/opentelemetry/context.rb

@@ -113,8 +113,11 @@ def value(key)
         current.value(key)
       end
 
+      # Clears the fiber-local Context stack. This allocates a new array for the
+      # stack, which is important in some use-cases to avoid sharing the backing
+      # array between fibers.
       def clear
-        stack.clear
+        Thread.current[STACK_KEY] = []
       end
 
       def empty