feat(nats/client): make stats extensible

Replace the stats dictionary by a class and interface, that is
extensible to allow users to replace the stats collection by their
own custom stats collector.

Resolves #720

Signed-off-by: Tim Drijvers <[email protected]>

Author: timdrijvers

nats/aio/client.py

@@ -52,6 +52,7 @@
 
 from .errors import ErrInvalidUserCredentials, ErrStaleConnection
 from .msg import Msg
+from .stats import ClientStats, StatsInterface
 from .subscription import (
     DEFAULT_SUB_PENDING_BYTES_LIMIT,
     DEFAULT_SUB_PENDING_MSGS_LIMIT,
@@ -127,6 +128,7 @@ class Srv:
     server_version: Optional[str] = None
 
 
+
 class ServerVersion:
 
     def __init__(self, server_version: str) -> None:
@@ -230,6 +232,7 @@ class Client:
     """
 
     msg_class: type[Msg] = Msg
+    stats: StatsInterface
 
     # FIXME: Use an enum instead.
     DISCONNECTED = 0
@@ -314,14 +317,7 @@ def __init__(self) -> None:
         self._public_nkey: Optional[str] = None
 
         self.options: Dict[str, Any] = {}
-        self.stats = {
-            "in_msgs": 0,
-            "out_msgs": 0,
-            "in_bytes": 0,
-            "out_bytes": 0,
-            "reconnects": 0,
-            "errors_received": 0,
-        }
+        self.stats = ClientStats()
 
     async def connect(
         self,
@@ -947,8 +943,7 @@ async def _send_publish(
             hdr.extend(_CRLF_)
             pub_cmd = prot_command.hpub_cmd(subject, reply, hdr, payload)
 
-        self.stats["out_msgs"] += 1
-        self.stats["out_bytes"] += payload_size
+        self.stats.message_sent(subject, payload_size, headers)
         await self._send_command(pub_cmd)
         if self._flush_queue is not None and self._flush_queue.empty():
             await self._flush_pending()
@@ -1510,7 +1505,7 @@ async def _attempt_reconnect(self) -> None:
 
                 # Consider a reconnect to be done once CONNECT was
                 # processed by the server successfully.
-                self.stats["reconnects"] += 1
+                self.stats.client_reconnected()
 
                 # Reset reconnect attempts for this server
                 # since have successfully connected.
@@ -1749,8 +1744,8 @@ async def _process_msg(
         Process MSG sent by server.
         """
         payload_size = len(data)
-        self.stats["in_msgs"] += 1
-        self.stats["in_bytes"] += payload_size
+        hdr = await self._process_headers(headers)
+        self.stats.message_received(subject.decode(), payload_size, hdr)
 
         sub = self._subs.get(sid)
         if not sub:
@@ -1764,8 +1759,6 @@ async def _process_msg(
             # internal queue and the task will finish once the last
             # message is processed.
             self._subs.pop(sid, None)
-
-        hdr = await self._process_headers(headers)
         msg = self._build_message(sid, subject, reply, data, hdr)
         if not msg:
             return

nats/aio/stats.py

@@ -0,0 +1,80 @@
+# Copyright 2016-2023 The NATS Authors
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from collections import UserDict
+from typing import Dict, Optional
+
+
+class StatsInterface(ABC):
+    """
+    Abstract base class defining the interface for NATS client statistics tracking.
+    """
+    
+    @abstractmethod
+    def message_received(self, subject: str, payload_size: int, headers: Optional[Dict[str, str]] = None) -> None:
+        """Record an incoming message with its payload size."""
+        pass
+    
+    @abstractmethod
+    def message_sent(self, subject: str, payload_size: int, headers: Optional[Dict[str, str]] = None) -> None:
+        """Record an outgoing message with its payload size."""
+        pass
+    
+    @abstractmethod
+    def client_reconnected(self) -> None:
+        """Record a client reconnection."""
+        pass
+    
+    @abstractmethod
+    def error_received(self) -> None:
+        """Record a server error."""
+        pass
+
+
+class ClientStats(StatsInterface, UserDict):
+    """
+    ClientStats tracks NATS client connection statistics and acts as a dict
+    for backward compatibility while providing structured methods for updates.
+    """
+    
+    def __init__(self) -> None:
+        super().__init__({
+            "in_msgs": 0,
+            "out_msgs": 0,
+            "in_bytes": 0,
+            "out_bytes": 0,
+            "reconnects": 0,
+            "errors_received": 0,
+        })
+    
+    def message_received(self, subject: str, payload_size: int, headers: Optional[Dict[str, str]] = None) -> None:
+        """Record an incoming message with its payload size."""
+        self.data["in_msgs"] += 1
+        self.data["in_bytes"] += payload_size
+    
+    def message_sent(self, subject: str, payload_size: int, headers: Optional[Dict[str, str]] = None) -> None:
+        """Record an outgoing message with its payload size."""
+        self.data["out_msgs"] += 1
+        self.data["out_bytes"] += payload_size
+    
+    def client_reconnected(self) -> None:
+        """Record a client reconnection."""
+        self.data["reconnects"] += 1
+    
+    def error_received(self) -> None:
+        """Record a server error."""
+        self.data["errors_received"] += 1