attributes now copy string data into internal storage instead of
storing pointers. this fixes segfaults when arena-allocated strings
are passed as span attributes - the data outlives the arena.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

track current span ID on thread so child spans can reference their
parent. when creating a span while another is active:
- capture current span as parent_span_id
- update tl_current_span_id to new span
- on span end, restore parent as current

adds parentSpanId to OTLP JSON output when present.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

concurrent requests on different thread pool workers were sharing
the same trace_id because current_trace_id was process-global.

now each thread has its own trace context via threadlocal:
- tl_trace_id: current trace ID for this thread
- tl_active_span_count: span nesting depth for this thread

this ensures each HTTP request gets its own trace_id even when
handled by different workers concurrently.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

the OTLP JSON format expects asInt/asDouble and histogram counts
to be numbers, not strings. only timestamps are strings.

compared against official examples in opentelemetry-proto repo.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

1. trace ID: generate new ID for each root span (when no active spans)
- add active_span_count tracking
- add newTrace() for explicit trace boundaries

2. metrics: change from cumulative to delta temporality
- each counter() call is an increment, not aggregated total

3. memory: track allocated NumberDataPoints and free on flush
- allocated_data_points list tracks allocations
- freed in flush() and deinit()

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flushes pending data every batch_timeout_ms (default 500ms).
stops cleanly on shutdown().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- metrics: Counter, Gauge, UpDownCounter, Histogram, ExponentialHistogram
- attributes: typed key-value pairs for spans/logs/metrics
- exporter: rewrite with std.json.Stringify (no manual string building)
- root: add counter(), gaugeInt(), gaugeDouble() convenience functions
- CLAUDE.md: project patterns and zig 0.15 notes

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>