feat: Add instrumentation decorator and apply to notes tools (Phase 5)

Created @instrument_tool decorator for automatic MCP tool metrics collection.
Applied to all 7 tools in notes.py.

Changes:
- observability/metrics.py:
  * New instrument_tool() decorator for automatic timing and error tracking
  * Compatible with @mcp.tool() and @require_scopes() decorators
  * Records tool_name, duration, and success/error status

- server/notes.py:
  * Applied @instrument_tool to all 7 tool functions
  * nc_notes_create_note, nc_notes_update_note, nc_notes_append_content
  * nc_notes_search_notes, nc_notes_get_note, nc_notes_get_attachment
  * nc_notes_delete_note

These metrics will populate the MCP Tool Calls dashboard panels.

Part of PR #295 - Complete metrics instrumentation (Phase 5)
Remaining: 86 tools across 8 server files

nextcloud_mcp_server/observability/metrics.py

@@ -395,3 +395,49 @@ def update_vector_sync_queue_size(size: int) -> None:
         size: Current queue size
     """
     vector_sync_queue_size.set(size)
+
+
+# =============================================================================
+# Decorator for Automatic Tool Instrumentation
+# =============================================================================
+
+
+def instrument_tool(func):
+    """
+    Decorator to automatically instrument MCP tool functions with metrics.
+
+    Wraps async tool functions to record execution time and success/error status.
+    Compatible with @mcp.tool() and @require_scopes() decorators.
+
+    Usage:
+        @mcp.tool()
+        @require_scopes("notes:write")
+        @instrument_tool
+        async def nc_notes_create_note(...):
+            ...
+
+    Args:
+        func: The async function to instrument
+
+    Returns:
+        Wrapped function with metrics instrumentation
+    """
+    import functools
+    import time
+
+    @functools.wraps(func)
+    async def wrapper(*args, **kwargs):
+        tool_name = func.__name__
+        start_time = time.time()
+        try:
+            result = await func(*args, **kwargs)
+            duration = time.time() - start_time
+            record_tool_call(tool_name, duration, "success")
+            return result
+        except Exception as e:
+            duration = time.time() - start_time
+            record_tool_call(tool_name, duration, "error")
+            record_tool_error(tool_name, type(e).__name__)
+            raise
+
+    return wrapper