Improve DSL

[DmitryTsepelev]

Changes:

• ability to group metrics with blocks:

  group :anycable do
  counter :rpc_executed_total
  end

• ability pass options to metrics with blocks:

  histogram :rpc_runtime do
  buckets [0.1, 10, 100]
  end

• friendly error messages about unknown/required options (option 'buckets' is required for Histogram, option 'wat' is not available for Histogram)

• tests are run in the random order