Cartridge role reload resets metrics data

[DifferentialOrange]

metrics package should support role reload, while it resets all observations and collectors.

Steps to reproduce

1. Create template application

   cartridge create --name test-app

2. Change init.lua

   local ok, err = cartridge.cfg({
       roles_reload_allowed = true,
       roles = {
           'cartridge.roles.vshard-storage',
           'cartridge.roles.vshard-router',
           'cartridge.roles.metrics',
           'app.roles.custom',
       },
   })

3. Start an application

   cartridge build
   cartridge start -d
   cartridge replicasets setup --bootstrap-vshard

4. Connect to instance

   cartridge enter router

5. Execute reload

   test-reload.router> metrics = require('metrics')
   ---
   ...
   
   test-reload.router> metrics.counter('c'):inc()
   ---
   ...
   
   test-reload.router> metrics.collect()
   ---
   - - label_pairs:
        alias: router
      timestamp: 1640183661554368
      value: 1
      metric_name: c
   ...
   
   test-reload.router> require("cartridge.roles").reload()
   ---
   - true
   ...
   
   test-reload.router> metrics = require('metrics')
   ---
   ...
   
   test-reload.router> metrics.collect()
   ---
   - []
   ...
   
   test-reload.router> metrics.collectors()
   ---
   - []
   ...
   

[Totktonada]

Why do you think that this is a problem?

[DifferentialOrange]

Why do you think that this is a problem?

I've discussed it with @yngvar-antonsson and he said this behavior is unexpected.

[yngvar-antonsson]

Reload of metrics package (without Cartridge) doesn't reset collectors, so cartridge hot reload doesn't have to reset them too.

[DifferentialOrange]

I think

package.loaded['cartridge.hotreload']
if hotreload ~= nil then
    hotreload.whitelist_globals({'__metrics_registry'})`
end

is the simplest and satisfying solution.