fix(logging): appengine zone label

[0xSage]

Fixes: #3919

Before:

resource: {
  type: "gae_app"
  labels: {
    project_id: "log-bench"
    module_id: "default"
    zone: ""
    version_id: "20201113t135702"
  }
}

After:

resource: {
  type: "gae_app"
  labels: {
    project_id: "log-bench"
    module_id: "default"
    zone: "us-west-a"
    version_id: "20201113t135702"
  }
}

e2e environment tests for this change passing at env-tests

=========================================== test session starts ===========================================
platform darwin -- Python 3.7.9, pytest-6.2.3, py-1.10.0, pluggy-0.13.1
rootdir: /Users/nicolezhu/Desktop/google-cloud-go/internal/env-tests-logging
collected 3 items

tests/go/test_appengine_standard.py ['/Users/nicolezhu/Desktop/google-cloud-go/internal/env-tests-logging/envctl/envctl', 'go', 'appengine_standard', 'verify']
['/Users/nicolezhu/Desktop/google-cloud-go/internal/env-tests-logging/envctl/envctl', 'go', 'appengine_standard', 'deploy']
['/Users/nicolezhu/Desktop/google-cloud-go/internal/env-tests-logging/envctl/envctl', 'go', 'appengine_standard', 'verify']
['/Users/nicolezhu/Desktop/google-cloud-go/internal/env-tests-logging/envctl/envctl', 'go', 'appengine_standard', 'trigger', 'simplelog', 'log_text="test_monitored_resource d3654c60-a3d4-11eb-bb1c-acde48001122"']
.['/Users/nicolezhu/Desktop/google-cloud-go/internal/env-tests-logging/envctl/envctl', 'go', 'appengine_standard', 'trigger', 'simplelog', 'log_text="test_receive_log de86ea86-a3d4-11eb-bb1c-acde48001122"']
..

====================================== 3 passed in 176.73s (0:02:56) ======================================
nox > Session tests(language='go', platform='appengine_standard') was successful.

[codyoss]

LGTM