[metricbeat] upgrade prometheus libraries to 3.x

[tommyers-elastic]

prometheus recently shipped v3, which is their first major version upgrade in a number of years.

we should upgrade to 3.x (library version 0.3x) as soon as possible to ensure we are getting the latest functionality and supporting the most up to date prometheus backends.

the change comes with several potentially breaking changes for consumers of prometheus as a library (metricbeat). we need to thourough audit our usage to ensure we do not pass on any breaking changes to existing customers.

migration guide here. as well as some existing migration work which did not address all potential breaking change concerns.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)