api: provide _VERSION module info

As a part of new module policy, each module must have version info
stored in module._VERSION. For now, metrics._VERSION a duplicate of
metrics.VERSION, but the latter is likely to be deprecated later.

Author: DifferentialOrange

metrics/init.lua

@@ -156,4 +156,5 @@ return {
     http_middleware = require('metrics.http_middleware'),
     collect = collect,
     VERSION = VERSION,
+    _VERSION = VERSION,
 }