OpenMetrics version header out-of-date / Support latest OpenMetrics format

[dmarmugi]

Hi, Looking at the Content-Type header

CONTENT_TYPE_LATEST = 'application/openmetrics-text; version=0.0.1; charset=utf-8'

The version=0.0.1 should be bumped. Looking at the spec:

The content type MUST be: application/openmetrics-text; version=1.0.0; charset=utf-8

I'm afraid I haven't been able to find any clear doc detailing changes between 0.0.1 and 1.0.0 so I don't know if it's as simple as just updating that line.

FWIW I've seen other places encountering the same trouble with the MUST restriction: prometheus/prometheus#9430, prometheus/client_java#702