Register /metrics endpoint when using --enablemetrics

[vicsufer]

Requirements

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Description of the Change

When -m or --enablemetrics flags an DEBUG_METRICS environment variable is set, this is to avoid a feature gate used in prometheus_flask_exporter to avoid registering /metrics endpoint when live reloading is enabled. FYI
Prom2Teams enabled live reloading in v2.0.4

Benefits

/metrics endpoint will be created even with live reloading enabled.

Possible Drawbacks

Future versions of prometheus_flask_exporter may give additional functionality to DEBUG_METRICS.

Applicable Issues

#201 /metrics server not working