Export Prometheus metrics

[greenhat]

Close #53 #266

This PR add exporting Prometheus metrics for various oracle and pool states. It unifies metrics served via REST API /poolHealth, /oracleHealth and Prometheus metrics in metrics.rs. Also, Grafana dashboard is added in /scripts/grafana_dashboard.json.

[coveralls]

Pull Request Test Coverage Report for Build 5242029742

• 4 of 247 (1.62%) changed or added relevant lines in 9 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-3.4%) to 55.556%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/oracle_state.rs	0	1	0.0%
core/src/address_util.rs	0	3	0.0%
core/src/box_kind/oracle_box.rs	2	6	33.33%
core/src/oracle_types.rs	0	10	0.0%
core/src/main.rs	0	13	0.0%
core/src/api.rs	0	16	0.0%
core/src/monitor.rs	0	66	0.0%
core/src/metrics.rs	0	130	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/src/api.rs	1	0%
core/src/main.rs	1	0%

Totals
Change from base Build 5153730120:	-3.4%
Covered Lines:	2070
Relevant Lines:	3726

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💛 - Coveralls