Race condition while using dockerized database, gorm and prometheus

## Description
I am using prometheus plugin and came accross this problem. Version used is: gorm.io/plugin/prometheus v0.0.0-20210507023802-dc84a49b85d1
In project our testing environment is set up in following way:
1. creating new gorm connection, on new dockerized database. Each test case creates new container 
2. attach prometheus plugin using db.Use()
3. Execute test case
4. Run next test

This connection is fine for 3-5 tests where SELECT/CREATE/UPDATE/DELETE queries are sent. Then the environment is somehow corrupted (either by using global variables) or by not using tear down functions for database connection (defer Close()).
Usage in out project:
```
        timeZoneLocation := url.QueryEscape(location.String()) // location is time.Location
        dsn := fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4,utf8&sql_mode=ANSI_QUOTES&parseTime=True&loc=%s", user, password, host, port, name, timeZoneLocation)
        db, err = gorm.Open(mysql.Open(dsn), &gorm.Config{
                Logger:         logger.Default.LogMode(loglevel),
                NamingStrategy: &modernTvNamingStrategy{},
        })  
...
        gormPrometheus := prometheus.New(prometheus.Config{
                DBName:          name,
                RefreshInterval: 25, 
                MetricsCollector: []prometheus.MetricsCollector{
                        &prometheus.MySQL{
                                VariableNames: []string{"Threads_running"},
                        },  
                },  
        })  
        err = db.Use(gormPrometheus)
...
```
Here is a part of stack trace, where race condition occured. Two tests cases are included:
```
[mysql] 2021/05/22 16:28:00 packets.go:123: closing bad idle connection: EOF

2021/05/22 16:28:00 gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/mysql.go:48 dial tcp [::1]:50111: connect: connection refused
[0.000ms] [rows:-] SHOW STATUS

2021/05/22 16:28:00 gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/mysql.go:51
[error] gorm:prometheus query error: dial tcp [::1]:50111: connect: connection refused
--- PASS: TestCreateRecordingTvShowEpisode (62348400.00s)
=== RUN   TestCreateRecordingTvShowEpisodeWithoutSeason
2021/05/08 16:28:00 connecting to db. host = localhost; port = 50118
==================
WARNING: DATA RACE
Read at 0x00c0012aaed0 by goroutine 33:
  time.Time.abs()
      /usr/lib/go/src/time/time.go:447 +0x106
  time.Time.date()
      /usr/lib/go/src/time/time.go:926 +0x4e
  time.Time.Date()
      /usr/lib/go/src/time/time.go:483 +0x409
  log.(*Logger).formatHeader()
      /usr/lib/go/src/log/log.go:112 +0x3ed
  log.(*Logger).Output()
      /usr/lib/go/src/log/log.go:179 +0x1fe
  log.(*Logger).Printf()
      /usr/lib/go/src/log/log.go:191 +0x6e
  gorm.io/gorm/logger.logger.Trace()
      /home/martinv/go/pkg/mod/gorm.io/gorm@v1.21.11/logger/logger.go:152 +0xd97
  gorm.io/gorm/logger.(*logger).Trace()
      <autogenerated>:1 +0x164
  gorm.io/gorm.(*processor).Execute()
      /home/martinv/go/pkg/mod/gorm.io/gorm@v1.21.11/callbacks.go:133 +0xd61
  gorm.io/gorm.(*DB).Rows()
      /home/martinv/go/pkg/mod/gorm.io/gorm@v1.21.11/finisher_api.go:440 +0xc7
  gorm.io/plugin/prometheus.(*MySQL).collect()
      /home/martinv/go/pkg/mod/gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/mysql.go:48 +0x7c
  gorm.io/plugin/prometheus.(*MySQL).Metrics.func1()
      /home/martinv/go/pkg/mod/gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/mysql.go:33 +0xb4

Previous write at 0x00c0012aaed0 by goroutine 125:
  [failed to restore the stack]

Goroutine 33 (running) created at:
  gorm.io/plugin/prometheus.(*MySQL).Metrics()
      /home/martinv/go/pkg/mod/gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/mysql.go:31 +0x267
  gorm.io/plugin/prometheus.(*Prometheus).Initialize.func1()
      /home/martinv/go/pkg/mod/gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/prometheus.go:76 +0x151
  sync.(*Once).doSlow()
      /usr/lib/go/src/sync/once.go:68 +0x127
  sync.(*Once).Do()
      /usr/lib/go/src/sync/once.go:59 +0x46
  gorm.io/plugin/prometheus.(*Prometheus).Initialize()
      /home/martinv/go/pkg/mod/gorm.io/plugin/prometheus@v0.0.0-20210507023802-dc84a49b85d1/prometheus.go:74 +0x206
  gorm.io/gorm.(*DB).Use()
      /home/martinv/go/pkg/mod/gorm.io/gorm@v1.21.11/gorm.go:436 +0xc1
  git.moderntv.eu/websystem/app-api/internal/database.NewConnection()
      /home/martinv/moderntv/app-api/internal/database/connection.go:66 +0xa0f
  git.moderntv.eu/websystem/app-api/internal/database/db_test_utils.(*DBTestCase).attachDB.func1()
      /home/martinv/moderntv/app-api/internal/database/db_test_utils/connection.go:246 +0x16d
  github.com/cenkalti/backoff/v4.RetryNotifyWithTimer()
      /home/martinv/go/pkg/mod/github.com/cenkalti/backoff/v4@v4.1.0/retry.go:55 +0x1b1
  github.com/cenkalti/backoff/v4.RetryNotify()
      /home/martinv/go/pkg/mod/github.com/cenkalti/backoff/v4@v4.1.0/retry.go:34 +0xfd
  github.com/cenkalti/backoff/v4.Retry()
      /home/martinv/go/pkg/mod/github.com/cenkalti/backoff/v4@v4.1.0/retry.go:28 +0xe5
  github.com/ory/dockertest/v3.(*Pool).Retry()
      /home/martinv/go/pkg/mod/github.com/ory/dockertest/v3@v3.7.0/dockertest.go:550 +0x100
  git.moderntv.eu/websystem/app-api/internal/database/db_test_utils.(*DBTestCase).attachDB()
      /home/martinv/moderntv/app-api/internal/database/db_test_utils/connection.go:242 +0x344
  git.moderntv.eu/websystem/app-api/internal/database/db_test_utils.(*DBTestCase).Initialize()
      /home/martinv/moderntv/app-api/internal/database/db_test_utils/connection.go:111 +0x195
  git.moderntv.eu/websystem/app-api/internal/database/db_test_utils.NewDBTestCase()
      /home/martinv/moderntv/app-api/internal/database/db_test_utils/connection.go:65 +0x399
  git.moderntv.eu/websystem/app-api/internal/test_utils.createDbTestCase()
      /home/martinv/moderntv/app-api/internal/test_utils/db.go:129 +0x77
  git.moderntv.eu/websystem/app-api/internal/test_utils.dbTestCaseWithoutLocking()
      /home/martinv/moderntv/app-api/internal/test_utils/db.go:119 +0xca
  git.moderntv.eu/websystem/app-api/internal/test_utils.DbTestCase()
      /home/martinv/moderntv/app-api/internal/test_utils/db.go:112 +0x10b
  git.moderntv.eu/websystem/app-api/internal/content/pvr_manager.TestCreateExistingBundle()
      /home/martinv/moderntv/app-api/internal/content/pvr_manager/bundle_create_test.go:21 +0x88
  testing.tRunner()
      /usr/lib/go/src/testing/testing.go:1259 +0x22f
  testing.(*T).Run·dwrap·21()
      /usr/lib/go/src/testing/testing.go:1306 +0x47

Goroutine 125 (running) created at:
  testing.(*T).Run()
      /usr/lib/go/src/testing/testing.go:1306 +0x726
  testing.runTests.func1()
      /usr/lib/go/src/testing/testing.go:1598 +0x99
  testing.tRunner()
      /usr/lib/go/src/testing/testing.go:1259 +0x22f
  testing.runTests()
      /usr/lib/go/src/testing/testing.go:1596 +0x7ca
  testing.(*M).Run()
      /usr/lib/go/src/testing/testing.go:1504 +0x9d1
  main.main()
      _testmain.go:73 +0x22b
==================
```
As it can be seen part of the writing stack is missing, but I dont know why it is stripping it off.
By stripping the MetricsCollector part from prometheus config, the race condition is not causing race condition. The problem could either lay in sending EOF by the database to the prometheus plugin or by dockerized setup of gorm database tear down functions.

I'll try to replicate it in playground but atm. I am not able to do it

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Race condition while using dockerized database, gorm and prometheus #57

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