Use /_prometheusMetrics*NoTS endpoints for external prometheus use case (for kv only)

/_prometheusMetrics and /_prometheusMetricsHighCard return metrics with timestamps now (as it allows us to safe disk space, see MB-45843). But we can't return timestamps in GET /metrics result as there might be a time difference between ns_server machine and external prometheus (additionally, other services doesn't return timestamps at this point, so we don't want the result of /metrics to contain mixed datapoints - with and without timestamps).

To handle that kv team created separate endpoints that will not return timestamps: /_prometheusMetricsNoTS and /_prometheusMetricsHighNoTS which ns_server needs to use when collecting data for GET /metrics.