Set 777 file permissions on UDS socket

The permissions of the socket created by aiohttp aren't as permissive as
the ones set by the real agent. (see
aio-libs/aiohttp#4155)

from @devinsba:

test agent:

```
cnb@my-app:/var/run/datadog$ ls -la
total 4
drwxr-xr-x 2 root root   60 Aug  1 21:24 .
drwxr-xr-x 1 root root 4096 Aug  1 21:24 ..
srwxr-xr-x 1 root root    0 Aug  1 21:24 apm.socket
```

real agent:

```
cnb@my-app:/var/run/datadog$ ls -la
total 4
drwxr-xr-x 2 root root   80 Aug  1 21:30 .
drwxr-xr-x 1 root root 4096 Aug  1 21:31 ..
srwx-w--w- 1 root root    0 Aug  1 21:30 apm.socket
```

ddapm_test_agent/agent.py

@@ -1,10 +1,12 @@
 import argparse
+import atexit
 import base64
 from collections import OrderedDict
 import json
 import logging
 import os
 import pprint
+import socket
 import sys
 from typing import Awaitable
 from typing import Callable
@@ -722,6 +724,14 @@ def main(args: Optional[List[str]] = None) -> None:
         print(_get_version())
         sys.exit(0)
 
+    apm_sock: Optional[socket.socket]
+    if parsed_args.trace_uds_socket is not None:
+        apm_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        # os.unlink(parsed_args.trace_uds_socket)
+        apm_sock.bind(parsed_args.trace_uds_socket)
+        os.chmod(parsed_args.trace_uds_socket, 0o777)
+        atexit.register(lambda: os.unlink(parsed_args.trace_uds_socket))
+
     if parsed_args.trace_request_delay is not None:
         log.info(
             "Trace request stall seconds setting set to %r.",
@@ -744,7 +754,7 @@ def main(args: Optional[List[str]] = None) -> None:
         trace_request_delay=parsed_args.trace_request_delay,
     )
 
-    web.run_app(app, path=parsed_args.trace_uds_socket, port=parsed_args.port)
+    web.run_app(app, sock=apm_sock, port=parsed_args.port)
 
 
 if __name__ == "__main__":

releasenotes/notes/uds-permissions-903266ac6445b873.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Use 777 permissions for UDS socket.