Add MTTM of security vulnerability into the centralized engineering metrics dashboard

Add MTTM of security vulnerabilities to the handbook

• Dashboard https://app.periscopedata.com/app/gitlab/758795/Appsec-Embedded-Dashboard
• Chart https://app.periscopedata.com/app/gitlab/758795/Appsec-Embedded-Dashboard?widget=9960552&udv=0

Tasks

• Break down MTTM by product groups / stages: https://app.periscopedata.com/app/gitlab/758795/Appsec-Embedded-Dashboard?widget=9960552&udv=0
• Add the product group / stages chart to the handbook
• Account for bugs with no product/group #9948 (comment 466532221)

cc @laurence.bierner @kwiebers @lmai1 @davis_townsend