feat: Output Formatters for Metrics (TASK-402)

[copyleftdev]

Problem

The existing output formatters in src/output.zig only support a basic TestResult struct with limited fields. The full Metrics structure from TASK-401 contains comprehensive data (requests, latency percentiles, throughput, connections, errors) that needs to be formatted for human-readable summaries, machine-readable JSON, CSV time-series, and comparison diffs.

Solution

Enhanced src/output.zig with formatters for the full Metrics structure:

New Formatters:

• formatSummaryText() - Human-readable summary with sections for requests, latency (ms), throughput (RPS), errors, and connections
• formatMetricsJSON() - Machine-readable JSON with pretty-print and compact modes, includes by_method and by_status breakdowns
• formatTimeSeriesHeader() / formatTimeSeriesRow() - CSV time-series format for streaming interval data
• formatDiff() - Compare two metric runs with improvement/regression indicators

Helper Functions:

• nsToMs() - Convert nanoseconds to milliseconds
• formatWithCommas() - Number formatting with thousand separators (e.g., 1,234,567)
• formatPercent() - Percentage formatting with 1 decimal place
• formatDelta() - Delta with direction indicator for comparisons

New Types:

• OutputConfig - Configuration struct for output options
• IntervalMetrics - Data structure for time-series data points

Convenience Functions:

• metricsToJSON() - Allocate JSON string from Metrics
• metricsToSummary() - Allocate summary string from Metrics

Testing

All 23 tests passing:

• 6 legacy TestResult tests (backward compatibility)
• 17 new Metrics formatter tests covering:
  • Helper function unit tests (nsToMs, formatWithCommas, formatPercent)
  • Summary text generation (basic and empty metrics)
  • JSON output structure and by_method breakdown
  • Compact JSON mode (no newlines)
  • CSV time-series header and row formatting
  • Diff output with improvements and regressions
  • Convenience function allocation tests

zig test src/output.zig
# All 23 tests passed.

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