refactor: metrics

[turip]

Overview

This patch improves the consistency/data available in consumer metrics. Changes:

• refactor: use int64 histograms (better for datadog, float64 ones are not working properly)
• refactor: add group handler metrics (So that we can report the skipped/ignored messages too.)
• refactor: unify router metrics (so that they are better aligned with the previous one)
• refactor: rename sarama metrics (Previously it was sarama.publisher.publisher / sarama.publisher.subscriber, now the redundant publisher part is dropped)
• refactor: use namespaces (e.g. watermill.router. for metrics, for easier debugging)