{Core} Extract User-Agent generation to get_az_user_agent

[jiasli]

Extract User-Agent generation to get_az_user_agent so that PR #12717 can also apply to az rest.

The logic to append custom User-Agent via AZURE_HTTP_USER_AGENT env var is removed from mgmt and data client factory, as this is already handled by msrest and causes duplication:

> $env:AZURE_HTTP_USER_AGENT='myua'
> az group list --debug
msrest.http_logger :     'User-Agent': 'python/3.8.2 (Windows-10-10.0.18362-SP0) msrest/0.6.1
1 myua msrest_azure/0.6.2 azure-mgmt-resource/8.0.1 Azure-SDK-For-Python AZURECLI/2.2.0 myua'

Note that myua appeared twice.

On the other hand, this logic is added to az rest to align with msrest. az rest previously can't handle AZURE_HTTP_USER_AGENT.

[jiasli]

These test methods are moved from the wrong class TestHandleException to the right class TestUtils.

[jiasli]

User-Agent for az rest is tested here.

[jiasli]

#12717 can use this test method to verify whether the installer is added to UA correctly.

[yonzhan]

add to S167

[jiasli]

Azure DevOps seems to be adding AZURE_HTTP_USER_AGENT=VSTS_0fb41ef4-5012-48a9-bf39-4ee3de03ee3 thus making the test fail:

FAIL: test_send_raw_requests (azure.cli.core.tests.test_util.TestUtils)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/mock/mock.py", line 1369, in patched
    return func(*newargs, **newkeywargs)
  File "/opt/hostedtoolcache/Python/3.6.10/x64/lib/python3.6/site-packages/azure/cli/core/tests/test_util.py", line 264, in test_send_raw_requests
    self.assertDictEqual(dict(request.headers), expected_header)
AssertionError: {'Use[38 chars]01937 VSTS_0fb41ef4-5012-48a9-bf39-4ee3de03ee3[208 chars]'12'} != {'Use[38 chars]01937', 'Accept-Encoding': 'gzip, deflate', 'A[154 chars]'12'}
  {'Accept': '*/*',
   'Accept-Encoding': 'gzip, deflate',
   'CommandName': 'rest',
   'Connection': 'keep-alive',
   'Content-Length': '12',
   'Content-Type': 'application/json',
   'ParameterSetName': 'method uri',
-  'User-Agent': 'AZURECLI/2.2.0.dev20200324101937 '
?                                                 -

+  'User-Agent': 'AZURECLI/2.2.0.dev20200324101937'}
?                                                  +

-                'VSTS_0fb41ef4-5012-48a9-bf39-4ee3de03ee35_build_246_0'}

Add @mock.patch.dict('os.environ') to manually remove it.

[Juliehzl]

May I know why we need the new function to handle agents? From my side, the previous UA definition should be fine, too.

[jiasli]

Because previously the functionality of concatenating UA is dispersed in 3 places:

• mgmt client
• data client
• az rest

It is difficult to maintain. I extrated the logic to one place so that changes done here automatically applies to all 3 places, given #12717 is doing more tweaks on the UA.

[Juliehzl]

LGTM in general.