Add metrics

[eminano]

This PR adds metrics to the core wal components using opentelemetry interfaces to extend compatibility with instrumentation implementations.

Added metrics include:

• wal processor lag (time it takes to process a message since it has been put into the replication stream)
• wal processor latency (time it takes to process a wal event)
• wal replication slot lag (lag accumulated by the consumer of the wal replication slot in bytes)
• kafka writer batch size/bytes (number and size of the messages being sent to kafka)
• kafka writer send latency (time it takes to send messages to kafka)

Commits have been split for ease of reviewing.