Add http end point observer

[gdey]

[observability] Split out httpAPI and httpViewer instrumentation

Split out the Observability interface so that API route and dealt with separately from the viewer routes.

Moved Null Observer to an internal package, treating nil as a Null Observer. Prometheus observer can be configured to indicate which API vars should be filled in. Default variables are :map_name, :layer_name and :z.

fixes #754

[coveralls]

Pull Request Test Coverage Report for Build 835b326f3-PR-755

• 9 of 32 (28.13%) changed or added relevant lines in 9 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 45.556%

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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/server.go	0	1	0.0%
cmd/tegola/cmd/cache/seed_purge.go	0	2	0.0%
maths/maths.go	0	3	0.0%
atlas/atlas.go	0	8	0.0%
server/server.go	5	13	38.46%
<!--	Total:	9	32	28.13%	-->

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

Totals
Change from base Build 671145575:	-0.04%
Covered Lines:	5413
Relevant Lines:	11882

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💛 - Coveralls

[ARolek]

@gdey could you add a section to the http package README.md called something like "Prometheus Metrics" and explain the metrics that are available from this package