code: func (c *requestContextMap) init(req *http.Request, context Context) bool {
  	c.lock.Lock()
  	defer c.lock.Unlock()
  	if _, exists := c.contexts[req]; exists {
  		return false
  	}
  	c.contexts[req] = context
  	return true
  }
docs: // init maps the request to the given context and returns true if there was no context associated with the request already.
  // if a context was already associated with the request, it ignores the given context and returns false.
  // init is intentionally unexported to ensure that all init calls are paired with a remove after a request is handled