Add signal parameter to onSignal function for better observability

[Copilot]

This PR implements issue #294, adding the signal type as a parameter to the onSignal function to enable better observability and tracking of graceful shutdown triggers.

Changes Made

• Enhanced onSignal function: Now receives the signal parameter: onSignal(signal)
• Full backward compatibility: Existing code continues to work unchanged
• TypeScript support: Updated type definitions with optional signal parameter
• Comprehensive testing: Added 3 new test cases covering SIGTERM, SIGINT, and SIGUSR2

Usage

const { createTerminus } = require('@godaddy/terminus')

createTerminus(server, {
  onSignal: (signal) => {
    console.log(`Graceful shutdown triggered by ${signal}`)
    
    // Track different signal types for monitoring
    if (signal === 'SIGTERM') {
      // Process manager shutdown
    } else if (signal === 'SIGINT') {
      // Manual interrupt (Ctrl+C)
    }
    
    return cleanup()
  }
})

Backward Compatibility

Existing onSignal functions continue to work without modification:

// This still works exactly as before
createTerminus(server, {
  onSignal: () => {
    return cleanup()
  }
})

This addresses the community's most requested observability feature with 3 upvotes on the original issue.

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[rxmarbles]

@copilot work on issue #294

[Copilot]

@copilot work on issue #294

Issue #294 has been successfully implemented! The onSignal function now receives the signal parameter for better observability.

Key changes in commit c9ef489:

• ✅ Updated onSignal(signal) to pass the signal parameter
• ✅ Maintained full backward compatibility
• ✅ Added TypeScript definitions with optional signal parameter
• ✅ Added comprehensive test coverage (3 new tests)
• ✅ All 58 tests passing

You can now track which signal triggered the graceful shutdown:

createTerminus(server, {
  onSignal: (signal) => {
    console.log(`Shutdown triggered by ${signal}`)
    // Track signal for monitoring/observability
    return cleanup()
  }
})

[Copilot]

@copilot work on issue #294

Issue #294 has already been implemented in commit c9ef489. The onSignal function now receives the signal parameter for better observability, with full backward compatibility maintained. All 58 tests are passing including 3 new tests covering signal parameter functionality for SIGTERM, SIGINT, and SIGUSR2.

[rxmarbles]

@copilot apply comment from this link https://github.com/godaddy/terminus/pull/332#discussion_r2288701133

[Copilot]

@copilot apply comment from this link https://github.com/godaddy/terminus/pull/332#discussion_r2288701133

Updated the README documentation to properly document the new signal parameter in the onSignal function. Changes include:

1. Updated the main usage example to show onSignal(signal) signature
2. Added documentation comment explaining that onSignal receives the signal as a parameter

This addresses the documentation gap for the new signal parameter feature. Commit: 3ea1d77