enable_default_metrics can't be enabled after metrics.clear

[olegrok]

tarantool> box.cfg{}
tarantool> metrics = require('metrics')
tarantool> metrics.enable_default_metrics()
tarantool> #metrics.collect()                               -- Seems to be ok
---
- 0
...

tarantool> metrics.invoke_callbacks()
tarantool> #metrics.collect()                                -- Ok. Got it after enable
---
- 64
...

tarantool> metrics.clear()
---
...

tarantool> metrics.invoke_callbacks()             
tarantool> #metrics.collect()                       
---
- 0
...

tarantool> metrics.enable_default_metrics()  -- Enable again
tarantool> metrics.invoke_callbacks()
tarantool> #metrics.collect()                           -- Where are my metrics? 
---
- 0
...

I expect last row returns me default metrics because I enabled them again.