[exporter-prometheus] Sanitize metric names in getPrometheusMetricName.

[songy23]

opencensus-node/packages/opencensus-exporter-prometheus/src/prometheus-stats.ts

Lines 158 to 160 in 097d1de

	private getPrometheusMetricName(view: View): string {
	return `${this.prefix}_${view.name}_${this.getUnit(view)}`;
	}

.

See the original discussion on #165 (comment).

We should replace all unsupported characters (e.g '/') with '_' for Prometheus metric names. Example in Go: https://github.com/census-instrumentation/opencensus-go/blob/master/internal/sanitize.go. Java: https://github.com/prometheus/client_java/blob/master/simpleclient/src/main/java/io/prometheus/client/Collector.java#L186-L196.

[mayurkale22]

Fixed in #217