Unable to create plain summary

[DifferentialOrange]

metrics 0.9.0

tarantool> metrics = require('metrics')
---
...

tarantool> metrics.summary('plain_summary')
---
- error: '...pp/.rocks/share/tarantool/metrics/collectors/summary.lua:21: bad argument
    #1 to ''pairs'' (table expected, got nil)'
...

It is impossible to create summary collector without specifying observations. If we want user to set up observation for each summary (which is weird since summary without quantiles is still useful), at least we should throw an understandable error message.