pin prometheus-client version to avoid bug

[kouk]

The latest ruby client for prometheus seems to be incompatible with this plugin:

/ $ gem list | grep promethe
fluent-plugin-prometheus (1.6.0)
prometheus-client (0.10.0)

I get this on fluentd startup:

2019-10-08 12:00:53 +0000 [error]: #0 unexpected error error_class=ArgumentError error="wrong number of arguments (given 2, expected 1; required keyword: docstring)"
  2019-10-08 12:00:53 +0000 [error]: #0 /usr/lib/ruby/gems/2.5.0/gems/prometheus-client-0.10.0/lib/prometheus/client/registry.rb:56:in `gauge'
  2019-10-08 12:00:53 +0000 [error]: #0 /usr/lib/ruby/gems/2.5.0/gems/fluent-plugin-prometheus-1.6.0/lib/fluent/plugin/in_prometheus_output_monitor.rb:69:in `start'
  2019-10-08 12:00:53 +0000 [error]: #0 /usr/lib/ruby/gems/2.5.0/gems/fluentd-1.6.3/lib/fluent/compat/call_super_mixin.rb:42:in `start'

I don't know enough ruby to resolve the hypothetical compatibility problems, but pinning the client version fixes the issue for me.

see: prometheus/client_ruby#95