Add amreceiver_alerts_total metric

[robertodauria]

This PR adds the amreceiver_alerts_total metric to keep track of alerts coming from AlertManager. This allows us to get historical data on which alerts fired/resolved and were routed to this receiver.

I've prefixed the metric with the amreceiver namespace in an attempt to follow https://prometheus.io/docs/practices/naming/.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 111

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 105:	0.0%
Covered Lines:	297
Relevant Lines:	297

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💛 - Coveralls