tools: Add traffic_grapher for real-time ATS metrics visualization (#12848)

* Add traffic_grapher.py, a Python tool that displays
  real-time graphs of RPS, latency, cache hit rate, and
  connections via JSONRPC Unix socket. Supports 1-4 hosts
  with multi-host comparison.

* Renders matplotlib charts inline in iTerm2 via imgcat
  with dark theme. Optional GUI mode with keyboard
  navigation (h/l or arrows) across 4 metric pages.

* Auto-discovers JSONRPC socket path on target hosts.
  Configurable refresh interval and history window.

Author: bryancall

tools/traffic_grapher/README.md

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+<!--
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you 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.
+-->
+
+# Traffic Grapher
+
+Real-time ATS metrics visualization for iTerm2. Displays live graphs of
+requests/sec, latency, cache hit rate, connections, and more — inline in
+your terminal using imgcat.
+
+## Features
+
+- Real-time graphs of RPS, latency, cache hit rate, connections
+- Support for 1–4 hosts with different line styles for comparison
+- Collects metrics via JSONRPC Unix socket (batch collection per host)
+- Dark theme optimized for terminal display
+- Keyboard navigation between 4 metric pages
+- Configurable refresh interval and history window
+- Optional GUI mode via matplotlib window
+
+## Requirements
+
+- Python 3.9+
+- [uv](https://docs.astral.sh/uv/) (recommended) or pip
+- iTerm2 (or compatible terminal for inline images)
+- SSH access to remote ATS hosts (not needed for localhost)
+
+## Quick Start
+
+```bash
+# Monitor ATS on the local machine (connects directly to JSONRPC socket)
+uv run traffic_grapher.py localhost
+
+# Monitor a remote host (connects via SSH)
+uv run traffic_grapher.py ats-server1.example.com
+
+# Multiple hosts for comparison
+uv run traffic_grapher.py ats-server{1..4}.example.com
+
+# Custom interval and history
+uv run traffic_grapher.py --interval 2 --history 120 ats-server1.example.com
+```
+
+## Installation (Alternative)
+
+```bash
+# Install as a project
+uv sync
+uv run traffic-grapher ats-server1.example.com
+
+# Or with pip
+pip install .
+traffic-grapher ats-server1.example.com
+```
+
+## Configuration
+
+### ATS Paths
+
+For localhost, the tool connects directly to the JSONRPC Unix socket.
+For remote hosts, it sends a Python script via SSH that connects to
+the socket on the remote machine.
+
+Configure the socket path via CLI flag or environment variable:
+
+| Option | Env Var | Default |
+|--------|---------|---------|
+| `--socket` | `TRAFFICSERVER_JSONRPC_SOCKET` | `/usr/local/var/trafficserver/jsonrpc20.sock` |
+
+### Custom Dashboards
+
+Create a YAML config file to customize which metrics are displayed:
+
+```bash
+uv run traffic_grapher.py -c my_dashboard.yaml ats-server1.example.com
+```
+
+## Keyboard Controls
+
+| Key | Action |
+|-----|--------|
+| `h` / `←` | Previous page |
+| `l` / `→` | Next page |
+| `q` | Quit |
+
+## Pages
+
+1. **Traffic & Cache** — Requests/sec, latency, cache hit rate, connections
+2. **Response Codes** — 2xx, 3xx, 4xx, 5xx breakdown
+3. **TLS & HTTP/2** — SSL handshakes, connections, HTTP/2 stats
+4. **Network & Errors** — Bandwidth, connection errors, transaction errors
+
+## Options
+
+```
+--interval SEC     Refresh interval in seconds (default: 1.0)
+--history SEC      History window in seconds (default: 60)
+--socket PATH      Path to JSONRPC Unix socket on remote host
+--gui              Use matplotlib GUI window instead of imgcat
+--once             Single snapshot, then exit
+--timezone TZ      Timezone for display (default: UTC)
+--save-png FILE    Save PNG after each render (use {iter} for iteration)
+--no-keyboard      Disable keyboard handling (for non-TTY environments)
+```

tools/traffic_grapher/pyproject.toml

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+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you 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.
+
+[project]
+name = "traffic-grapher"
+version = "1.0.0"
+description = "Real-time ATS metrics visualization for iTerm2"
+requires-python = ">=3.9"
+license = "Apache-2.0"
+dependencies = [
+    "matplotlib>=3.7",
+    "pyyaml>=6.0",
+]
+
+[project.scripts]
+traffic-grapher = "traffic_grapher:main"