feat: add plugin execution tracing to OpenTelemetry plugin

[aliibii]

Description
This commit adds plugin execution tracing capability to the OpenTelemetry plugin, allowing users to trace individual plugin phases (rewrite, access, header_filter, body_filter, log) as child spans of the main request trace.

Changes:

• Added trace_plugins configuration object with the following properties:
  • enabled: boolean to enable/disable plugin execution tracing (default: false)
  • plugin_span_kind: string enum ("internal" or "server") for observability provider compatibility (default: "internal")
  • excluded_plugins: array of plugin names to exclude from tracing (default: ["opentelemetry", "prometheus"])
• Enhanced plugin execution in plugin.lua with OpenTelemetry span creation and finishing
• Added proper span hierarchy: plugin phase spans are nested under main request spans
• Added span context management with stack-based tracking for nested spans
• Added upstream attributes (upstream.addr, upstream.host, upstream.ip, upstream.port) to the main request span in before_proxy phase
• Updated test suite to reflect new schema structure and added tests for new features

New OpenTelemetry API functions available to plugins via api_ctx.otel:

• api_ctx.otel.start_span(span_info) - Create custom spans

  • Parameters: span_info table with optional fields: name, kind, attributes, parent
  • Returns: span context object or nil
  • Automatically tracks spans in a stack for proper parent-child relationships

• api_ctx.otel.stop_span(span_ctx, error_msg) - Finish spans with error handling

  • Parameters: span_ctx (from start_span), optional error_msg string
  • Sets error status on span if error_msg provided
  • Automatically manages span stack

• api_ctx.otel.current_span() - Get current span context

  • Returns: most recently started span context (top of stack) or nil
  • Useful for adding attributes or creating child spans

• api_ctx.otel.get_plugin_context(plugin_name, phase) - Get plugin phase span context

  • Parameters: plugin_name (string), phase (string)
  • Returns: span context for the specified plugin phase or nil
  • Useful for plugins that want to reference or extend existing plugin phase spans

• api_ctx.otel.with_span(span_info, fn) - Create span, execute function, and automatically finish span

  • Parameters: span_info table (same as start_span), fn function to execute
  • The function receives span_ctx as its first parameter, allowing access to the span for setting attributes
  • Automatically handles span creation, execution, error handling, and cleanup
  • Returns function results in error-first pattern (err, ...values)
  • Sets span status to ERROR if function throws Lua error or returns an error

Features:

• Plugin Phase Tracing: Creates child spans for each plugin phase execution automatically
• Span Kind Control: Supports "internal" (default) and "server" span kinds for observability provider compatibility
• Configurable: Can be enabled/disabled via trace_plugins.enabled configuration
• Plugin Exclusion: Can exclude specific plugins from tracing (e.g., opentelemetry, prometheus)
• Proper Hierarchy: Plugin spans are correctly nested under main request spans
• Upstream Attributes: Upstream information (addr, host, ip, port) is automatically attached to the main request span
• Error Handling: Proper error status and message propagation for plugin execution errors
• No-op API: API is always available but returns no-ops when tracing is disabled
• Stack Management: Automatic span stack tracking for nested span hierarchies
• Convenient Span Management: with_span provides automatic span lifecycle management with error handling

Which issue(s) this PR fixes:

Resolves #12510

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[juzhiyuan]

1. Just approved to run tests.
2. @bzp2010 @moonming @Baoyuantop can have a look?