NIFI-9587 Add json output for Prometheus metrics

[timeabarna]

https://issues.apache.org/jira/browse/NIFI-9587

Description of PR

Enables exposing Prometheus metrics in json
Enables basic filtering for Sample names and Sample values

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[timeabarna]

Hello @exceptionfactory , as this PR is building on your PR 5571 would you mind reviewing it? Thanks in advance.

[exceptionfactory]

Thanks for putting together this feature @timeabarna! The implementation looks straightforward, and aligns well with the recently implemented filtering. I noted a couple minor questions, but otherwise it looks good.

[timeabarna]

Thanks @exceptionfactory for the review.

[exceptionfactory]

Thanks for making the adjustments @timeabarna, looks good! +1 Merging.