Add cache observer

[gdey]

Add Observability metrics for Cache Providers.

This will work for all providers. Includes support for push.

fixes #756

[gdey]

I have not had a chance to test the push on this. So, this is not quite ready.

[coveralls]

Pull Request Test Coverage Report for Build d74533b06-PR-766

• 9 of 69 (13.04%) changed or added relevant lines in 11 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 45.444%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/tegola/cmd/cache/cache.go	0	1	0.0%
cmd/tegola/cmd/version.go	0	1	0.0%
cmd/tegola/cmd/root.go	0	3	0.0%
maths/maths.go	0	3	0.0%
cmd/tegola/cmd/server.go	0	5	0.0%
cmd/tegola/cmd/cache/seed_purge.go	0	6	0.0%
server/server.go	5	13	38.46%
atlas/atlas.go	0	33	0.0%
<!--	Total:	9	69	13.04%	-->

Files with Coverage Reduction	New Missed Lines	%
maths/maths.go	1	30.87%
atlas/atlas.go	3	3.88%
<!--	Total:	4		-->

Totals
Change from base Build 671145575:	-0.2%
Covered Lines:	5416
Relevant Lines:	11918

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💛 - Coveralls