Revert api change from #2086 introduce Puma.stats_hash api

The change in #2086 is not backwards compatible with existing gems that parse the output of Puma.stats such as barnes.

Releasing a version of puma with this change would break anyone using the Barnes app and only in production. I'm proposing to keep the existing interface and instead add a new API. This buys us all the features of #2086 without causing any production facing downtime by customers due to API incompatibilities.

Unfortunately it requires that we serialize and the de-serialize the values. One prior benefit of returning json in a string was that it allowed an end user to de-serialize using a faster json algorithm such as `oj` via the "multi json" gem. But the performance penalty will be better than a stability break.

History.md

@@ -13,7 +13,7 @@
   * New configuration option to set state file permissions (#2238)
 
 * Deprecations, Removals and Breaking API Changes
-  * `Puma.stats` now returns a Hash instead of a JSON string (#2086)
+  * `Puma.stats_hash` returns a Hash version of the stats (#2086, )
   * `--control` has been removed. Use `--control-url` (#1487)
   * `worker_directory` has been removed. Use `directory`.
   * min_threads now set by environment variables PUMA_MIN_THREADS and MIN_THREADS. (#2143)

lib/puma.rb

@@ -20,7 +20,11 @@ def self.stats_object=(val)
   end
 
   def self.stats
-    @get_stats.stats
+    @get_stats.stats.to_json
+  end
+
+  def self.stats_hash
+    @get_stats.stats.to_json
   end
 
   # Thread name is new in Ruby 2.3