[Perf] keyvault-secrets test improvements (Azure#20900)

- Rename option from "list-size" to "count" to align with other languages
- Validate vault contains 0 secrets before running

sdk/keyvault/azure-keyvault-secrets/tests/perfstress_tests/list_secrets.py

@@ -24,10 +24,19 @@ def __init__(self, arguments):
         vault_url = self.get_from_env("AZURE_KEYVAULT_URL")
         self.client = SecretClient(vault_url, self.credential, **self._client_kwargs)
         self.async_client = AsyncSecretClient(vault_url, self.async_credential, **self._client_kwargs)
-        self.secret_names = ["livekvtestlistperfsecret{}".format(i) for i in range(self.args.list_size)]
+        self.secret_names = ["livekvtestlistperfsecret{}".format(i) for i in range(self.args.count)]
 
     async def global_setup(self):
         """The global setup is run only once."""
+        # Validate that vault contains 0 secrets (including soft-deleted secrets), since additional secrets
+        # (including soft-deleted) impact performance.
+        async for secret in self.async_client.list_properties_of_secrets():
+            raise Exception("KeyVault %s must contain 0 secrets (including soft-deleted) before starting perf test" \
+                % self.async_client.vault_url)
+        async for secret in self.async_client.list_deleted_secrets():
+            raise Exception("KeyVault %s must contain 0 secrets (including soft-deleted) before starting perf test" \
+                % self.async_client.vault_url)
+
         await super().global_setup()
         create = [self.async_client.set_secret(name, "secret-value") for name in self.secret_names]
         await asyncio.wait(create)
@@ -63,5 +72,5 @@ async def run_async(self):
     def add_arguments(parser):
         super(ListSecretsTest, ListSecretsTest).add_arguments(parser)
         parser.add_argument(
-            '--list-size', nargs='?', type=int, help='Number of secrets to list. Defaults to 10', default=10
+            '--count', nargs='?', type=int, help='Number of secrets to list. Defaults to 10', default=10
         )