Cleanup CompactionProcessor metrics upon subscription close

[ocadaruma]

refs: #95 (comment)

• In principle, we close every Decaton's internal metrics by implementing https://github.com/line/decaton/blob/master/processor/src/main/java/com/linecorp/decaton/processor/metrics/Metrics.java#L64
• But CompactionProcessor is an exception
  • Since there's no way to pass subscriptionId upon CompactionProcessor instantiation, CompactionProcessor's metrics are registered every time per task, so AbstractMetrics isn't useable
    • Though we confirmed it doesn't cause much overhead
• Possible fix for this will be like below:
  • Make CompactionProcessor constructor to receive subscriptionId
    • Or It'd better to pass information which is available on the processor scope (partition, threadId) in addition to subscriptionId
  • Add ProcessorsBuilder#thenProcess overload that receives supplier of type Function<ScopeInfo, DecatonProcessor>