…ug info

**Which issue(s) this PR fixes**:
Fixes #

**What this PR does / why we need it**:
This PR updates the default behavior of the `monitor_agent` API to provide a more minimal and predictable response footprint.

### Before
The HTTP API (`/api/plugins.json`, etc.) included `config`, `retry` state, and `instance_variables` of plugins by default (or when dynamically queried with parameters like `debug=1`, `with_config`, `with_retry`, or `with_ivars`).

This resulted in unnecessarily verbose payloads that exposed deep internal plugin states not typically required for standard health monitoring.

### After
Inclusion of these detailed internal states is now disabled by default to reduce payload size and enforce explicit configuration.

* Changed
  * `include_config` default is changed from `true` to `false`.
  * `include_retry` default is changed from `true` to `false`.
  * The API no longer allows URL query parameters to override these settings.
* Added
  * `include_debug_info` parameter to explicitly control the visibility of internal instance variables (default `false`).
  * Debug information (via queries such as `debug=1` or `with_ivars=xxx`) is now only included when this parameter is explicitly enabled in the configuration block.

**Docs Changes**:

**Release Note**:
* in_monitor_agent: change default visibility of config, retry, and debug info

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>