chore: import upstream snapshot with attribution

contributing/samples/plugin/plugin_basic/README.md

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+# ADK Agent with Plugin
+
+### What is ADK Plugin?
+
+At its core, ADK extensibility is built on
+[**callbacks**](https://google.github.io/adk-docs/callbacks/): functions you
+write that ADK automatically executes at key stages of an agent's lifecycle.
+**A Plugin is simply a class that packages these individual callback functions
+together for a broader purpose.**
+
+While a standard Agent Callback is configured on a *single agent, a single tool*
+for a *specific task*, a Plugin is registered *once* on the `Runner` and its
+callbacks apply *globally* to every agent, tool, and LLM call managed by that
+runner. This makes Plugins the ideal solution for implementing horizontal
+features that cut across your entire application.
+
+### What can plugins do?
+
+Plugins are incredibly versatile. By implementing different callback methods, you
+can achieve a wide range of functionalities.
+
+- **Logging & Tracing**: Create detailed logs of agent, tool, and LLM activity
+  for debugging and performance analysis.
+- **Policy Enforcement**: Implement security guardrails. For example, a
+  before_tool_callback can check if a user is authorized to use a specific
+  tool and prevent its execution by returning a value.
+- **Monitoring & Metrics**: Collect and export metrics on token usage,
+  execution times, and invocation counts to monitoring systems like Prometheus
+  or Stackdriver.
+- **Caching**: In before_model_callback or before_tool_callback, you can
+  check if a request has been made before. If so, you can return a cached
+  response, skipping the expensive LLM or tool call entirely.
+- **Request/Response Modification**: Dynamically add information to LLM prompts
+  (e.g., in before_model_callback) or standardize tool outputs (e.g., in
+  after_tool_callback).
+
+### Run the agent
+
+**Note: Plugin is NOT supported in `adk web`yet.**
+
+Use following command to run the main.py
+
+```bash
+python3 -m contributing.samples.plugin_basic.main
+```
+
+It should output the following content. Note that the outputs from plugin are
+printed.
+
+```bash
+[Plugin] Agent run count: 1
+[Plugin] LLM request count: 1
+** Got event from hello_world
+Hello world: query is [hello world]
+** Got event from hello_world
+[Plugin] LLM request count: 2
+** Got event from hello_world
+```