change slice metrics to map metrics

[snowlyg]

I change MetricsList []*Metric to MetricsList map[string]*Metric, when i use the custom metrics. Make a easy way to get custom metric.

customMetrics := []*ginprometheus.Metric{
	&ginprometheus.Metric{
		ID:	"1234",				// optional string
		Name:	"test_metric",			// required string
		Description:	"Counter test metric",	// required string
		Type:	"counter",			// required string
	},
	&ginprometheus.Metric{
		ID:	"1235",				// Identifier
		Name:	"test_metric_2",		// Metric Name
		Description:	"Summary test metric",	// Help Description
		Type:	"summary", // type associated with prometheus collector
	},
	// Type Options:
	//	counter, counter_vec, gauge, gauge_vec,
	//	histogram, histogram_vec, summary, summary_vec
}
p := ginprometheus.NewPrometheus("gin", customMetrics)
// add data to custom metrics
m := p.MetricsList["1234"].MetricCollector.(prometheus.Counter)
m.Inc()

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