title: Multiple custom metric hits issue with GTM/GA transaction
body: <p><br>
  I track eCommerce transactions with GA and GTM, and trying to create a custom metric which will count how many people are involved in each transaction. 
  So I created a hit-scoped custom metric and try to pass its value along with the transaction tag in GTM:</p>
  
  <p>Please see screenshot here: </p>
  
  <p><a href="https://i.stack.imgur.com/Ht0cy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ht0cy.png" alt="Adding a custom metric to transaction hit"></a></p>
  
  <p><br><br>
  As we know, a transaction in GA is a series of hits: one "transaction" hit and one or more "item" hits. The problem is that GTM sends the custom metric value with ALL of the hits, not just with one. So when I use my hit-scoped metric to measure something, it's measured more than once. <br>
      For ex., in a transaction where 2 people are involved, this value (2) is send twice (once with "transaction" hit, once with "item" hit) and in the GA reports I see figure 4 (instead of 2) associated with this transaction. <br><br>
  I didn't find a way to ask GTM to send the custom metric just once.<br>
  Am I missing something here? Any suggestions?<br>
  Thanks!</p>