Log slow requests as warnings

[emostov]

In order to help Sidecar maintainers and sysadmins better trouble shoot issues with the service and its deployment it may be helpful to log slow requests at the warn log level (or log an additional message at the warn log level with every slow request).

rel: #368

[danforbes]

Could we allow Sidecar admins to configure what is considered a "slow" request? Is there existing Express middleware or something that handles this?

[TarikGul]

Our recent benchmarks of all the endpoints have shown that consumption of resources vastly vary across each service, therefore it's harder to track what is "slow" and what isn't. I would argue a better source of truth should be the benchmarks and we work on performance regressions from there. Closing.