KAFKA-13243: KIP-773 Differentiate metric latency measured in ms and ns
Implementation of KIP-773
Deprecates inconsistent metrics bufferpool-wait-time-total,
io-waittime-total, and iotime-total.
Introduces new metrics bufferpool-wait-time-ns-total,
io-wait-time-ns-total, and io-time-ns-total with the same semantics as
before.
Adds metrics (old and new) in ops.html.
Adds upgrade guide for these metrics.
Reviewers: Guozhang Wang <wangguoz@gmail.com>, Tom Bentley <tbentley@redhat.com>