[Fix] Exclude localhost from hitting proxies for metadata service req…

…uests (#877)

## What changes are proposed in this pull request?

Exclude localhost from hitting proxies for metadata service requests

`requests` package doesn't do it by default.

Relevant issue:
databricks/databricks-vscode#916 (comment)

## How is this tested?

Tested manually by running this code against real metadata service of
the logged in databricks vscode extension:
```python
import requests
import os

os.environ["HTTP_PROXY"] = "http://test.com"

resp = requests.get("http://127.0.0.1:53468/redacted",
                    timeout=10000,
                    headers={
                        "X-Databricks-Metadata-Version": "1",
                        "X-Databricks-Host": "https://redacted.databricks.com/"
                    }, proxies={"no_proxy": "localhost,127.0.0.1"})

print(resp.text)
``` 
The code fails without the no_proxy option

databricks/sdk/credentials_provider.py

@@ -676,12 +676,18 @@ def __init__(self, cfg: 'Config'):
         self.host = cfg.host
 
     def refresh(self) -> Token:
-        resp = requests.get(self.url,
-                            timeout=self._metadata_service_timeout,
-                            headers={
-                                self.METADATA_SERVICE_VERSION_HEADER: self.METADATA_SERVICE_VERSION,
-                                self.METADATA_SERVICE_HOST_HEADER: self.host
-                            })
+        resp = requests.get(
+            self.url,
+            timeout=self._metadata_service_timeout,
+            headers={
+                self.METADATA_SERVICE_VERSION_HEADER: self.METADATA_SERVICE_VERSION,
+                self.METADATA_SERVICE_HOST_HEADER: self.host
+            },
+            proxies={
+                # Explicitly exclude localhost from being proxied. This is necessary
+                # for Metadata URLs which typically point to localhost.
+                "no_proxy": "localhost,127.0.0.1"
+            })
         json_resp: dict[str, Union[str, float]] = resp.json()
         access_token = json_resp.get("access_token", None)
         if access_token is None: