Fix flakiness of test_metric_collection (#4346)

## Problem

Test `test_metric_collection` become flaky:

```
AssertionError: assert not ['2023-05-25T14:03:41.644042Z ERROR metrics_collection: failed to send metrics: reqwest::Error { kind: Request, url: Url { scheme: "http", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("localhost")), port: Some(18022), path: "/billing/api/v1/usage_events", query: None, fragment: None }, source: hyper::Error(Connect, ConnectError("tcp connect error", Os { code: 99, kind: AddrNotAvailable, message: "Cannot assign requested address" })) }',
                            ...]
```
I suspect it is caused by having 2 places when we define
`httpserver_listen_address` fixture (which is internally used by
`pytest-httpserver` plugin)

## Summary of changes
- Remove the definition of `httpserver_listen_address` from
`test_runner/regress/test_ddl_forwarding.py` and keep one in
`test_runner/fixtures/neon_fixtures.py`
- Also remote unused `httpserver_listen_address` parameter from
`test_proxy_metric_collection`

test_runner/regress/test_ddl_forwarding.py

@@ -4,21 +4,12 @@ from typing import Any, Dict, List, Optional, Tuple, Type
 import psycopg2
 import pytest
 from fixtures.log_helper import log
-from fixtures.neon_fixtures import (
-    PortDistributor,
-    VanillaPostgres,
-)
+from fixtures.neon_fixtures import VanillaPostgres
 from pytest_httpserver import HTTPServer
 from werkzeug.wrappers.request import Request
 from werkzeug.wrappers.response import Response
 
 
-@pytest.fixture(scope="session")
-def httpserver_listen_address(port_distributor: PortDistributor):
-    port = port_distributor.get_port()
-    return ("localhost", port)
-
-
 def handle_db(dbs, roles, operation):
     if operation["op"] == "set":
         if "old_name" in operation and operation["old_name"] in dbs:

test_runner/regress/test_metric_collection.py

@@ -228,7 +228,6 @@ def proxy_with_metric_collector(
 @pytest.mark.asyncio
 async def test_proxy_metric_collection(
     httpserver: HTTPServer,
-    httpserver_listen_address,
     proxy_with_metric_collector: NeonProxy,
     vanilla_pg: VanillaPostgres,
 ):