[keyserver] Introduce logEndpointMetrics and yarn dev-log-metrics

Summary:
Introduce logEndpointMetrics middleware (on Express middleware: https://expressjs.com/en/guide/using-middleware.html) to log request-response duration and Content-Length for each endpoint.

This middleware is enabled when KEYSERVER_ENDPOINT_METRICS_ENABLED is set.

Will introduce a separate yarn command to write logs to disk

Test Plan:
Logs look as expected when KEYSERVER_ENDPOINT_METRICS_ENABLED is set:

Logs don't appear when KEYSERVER_ENDPOINT_METRICS_ENABLED is unset.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D9308