Add TTFB Min and Max to k6 Results

Adds the ability to pass in GPT_TTFB_MIN and GPT_TTFB_MAX environment variables to allow for min and max metrics to be reported in the test results and available for InfluxDB as well.

The Workspace team is interested in these metrics for the linear queries work being done in Track Improvements for Linear Namespace Queries (gitlab-org&7922 - closed) (please also see this comment)