fix(context): do not modify stack array directly when attach and detach

[tientt-holistics]

As mentioned in open-telemetry/opentelemetry-ruby/pull/1598

As identified in open-telemetry/opentelemetry-ruby-contrib#772, Rails copies all thread-local variables in ActionController::Live. Our use of an array to back the Context stack results in two threads sharing the same stack. Context.clear only clears the array, but continues to share it (in fact, its effect is visible to both threads), so there is no way to cleanly separate the thread's Context stacks using the public API.

This PR will replace the stack.push and stack.pop with Thread.current assignment

[linux-foundation-easycla]

• ✅login: tientt-holistics / (cd841ef)

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