Add stubs for the absent telemetry endpoints when compiled without `telemetry` feature

[0x009922]

Currently, when irohad (and iroha_torii) are compiled without telemetry feature, the telemetry endpoints are completely excluded from the server (/metrics, /status, /peers).

I propose to have these endpoints anyway, but return 501 Not Implemented with a text message like

This endpoint is not available on this version of irohad, as it was compiled without the telemetry feature flag.

Benefits of it:

• Better troubleshooting for infra admins
• Ability to handle this case precisely programmatically. For example, when Explorer tries to gather telemetry from a peer that doesn't support it, it would do so by checking the exact error message returned, instead of some not-so-explicit 404 Not Found.