Hey @Takaaki 

Depending on how the customer metafield is being updated there is a good solution to get this data in to Klaviyo using Klaviyo’s Track Event Function in Shopify flow.

Unfortunately there is not a customer updated trigger in flow at present but if you can populate the metafield with the flow property can be pushed over to klaviyo as a customer property or event.

Below is an example for on order creation, populating customer metafield and sending data to Klaviyo.

The process for this is as follows:
1. Setting Up Shopify Flow

• Click Create Workflow and select the trigger: Shopify > Order is created.
• Then add an action: Flow > Wait > 1 minute (to allow backend processing).
• Next, add a Action: Update customer metafield 
• Populate the metafield data as you like. eg custom.age: 30

2. Connecting Shopify Flow with Klaviyo

For each of the actions previously defined, you'll now integrate Klaviyo to track events:

1. Click Then > Action > Klaviyo > Track Event.
2. Inside this event:

• Add your Klaviyo Public API Key.
• Set the Event Name for Klaviyo.
• For Customer Email, click Add Variable > Order > Email. This will insert the value {{order.email}}.
• For Customer First Name, click Add Variable > Order > Customer > First Name ({{order.customer.firstName}}).
• For Customer Last Name, click Add Variable > Order > Customer > Last Name ({{order.customer.lastName}}).

For Customer Properties (e.g., for "self" buyers), use:

{

  "customer-age": "{{variable}}."

}

For Event Properties, you would follow the same format
 

3. Setup Variable

Click between the quotation marks ("") and then click Add which ever variable information you wold need.

 4. Run a test order

Run a test order using the profile information you’ve set up. Once the order is completed, navigate to Klaviyo and search for the customer profile associated with the test. Check that the flow was triggered correctly and that the Customer Properties have been updated with the defined values, such as "customer-age: 30", if applicable.

Any issues or questions, let me know. 
Or if you are populating in another way.

Thanks

Tim