Add configurable logger, metrics

[squeed]

This adds structured logging via the go-logr interface "standard".

This also adds some optional Prometheus metrics:
- number of update messages
- number of updates per table
- number of disconnects
- number of monitors

[coveralls]

Pull Request Test Coverage Report for Build 1333446488

• 72 of 95 (75.79%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 73.001%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
client/client.go	21	23	91.3%
client/metrics.go	47	57	82.46%
client/options.go	4	15	26.67%

Totals
Change from base Build 1321747633:	0.2%
Covered Lines:	3999
Relevant Lines:	5478

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💛 - Coveralls

[dave-tucker]

@squeed the logging changes look good, but I'm wondering whether perhaps we could split out the metrics changes and combine with #146 as that implements tracing too

Edit: PR is #189

[dave-tucker]

rebased. if CI passes I'll pull this in.. will add some plumbing to bring logr to the cache/api layers and add some debug/trace level logging in another PR