Updating plugin to support latest prometheus ruby client version

[AntoineC44]

Hello,

I would like to implement the following feature of the ruby prom client: init_label_set that would allow metric init if specified by user in fluent.conf.

Though current fluent-plugin-prometheus build is blocked to old versions <0.10 of prometheus ruby client. init_label_set method is only available in recent versions.

This PR thus aims at using latest prom client version.

In detail

• fixing method signatures to match the recent versions of prometheus client
• fixing now mandatory declaration of labels at initialization of metrics
• updating tests to reflect changes
• updating internal monitoring plugins to reflect change

Please note the loss of summary metric quantiles as removed in this Pull Request on prometheus-client repo This effectively means that any summary metric will no longer display quantile labels (e.g. quantile="0.99") but only sum and count. Though personal experience on production environments showed anyway fluentd performance issues when over-using summary quantile computations. Prefer thus histograms instead - with custom buckets if needed - and computation of quantiles on prometheus server side. See the prometheus official documentation on this topic here.

[AntoineC44]

Looking for some feedback before giving sign-off All tests passed + redid some tests in local

[AntoineC44]

Hello @repeatedly, is this ok for you?

[repeatedly]

Sorry for the delay. Just merged! I will release new version soon with major version update.