Explore Chat Metrics - Individual Agent Reporting

I'm looking to report on the two items below:

1. Chat average handle time per agent (but I only want the duration from once the agent has accepted the assignment to the end of the chat)
2. Average First Reply time per once (again only once the agent has accepted the assignment – how long it takes the agent to respond to the customer after they’ve accepted the assignment).

I’ve been played around with different metrics in Explore but they all seem to start the time tracker at the customer/visitor’s first message. This is problematic because it does not give a true picture of average handle time per agent.