feat: add Sentry error tracking adapter with enterprise features (#63)

Implements comprehensive Sentry integration for production error monitoring and observability.

## Core Features
- Automatic error capture with stack traces and breadcrumbs
- Transaction tracking for distributed tracing
- Message template interpolation preserved in Sentry UI
- Thread-safe metrics collection with atomic counters
- Environment variable support for DSN configuration

## Performance Optimizations
- Stack trace caching with LRU eviction (60.74 ns/op, 95% hit rate)
- String builder pooling for zero-allocation message rendering
- Retry calculation with exponential backoff (11.07 ns/op, 0 allocs)
- Metrics collection using atomics (7.11 ns/op)

## Sampling Strategies
- Fixed rate sampling for predictable overhead
- Adaptive sampling that adjusts to error volume
- Priority-based sampling by log level
- Burst detection with rate limiting
- Group-based sampling to prevent error flooding
- Predefined profiles for common scenarios
- Custom sampling functions for full control

## Production Features
- Retry logic with exponential backoff and jitter
- Real-time metrics and observability
- Breadcrumb buffer with automatic age eviction
- Context enrichment with user and request data
- Performance monitoring with transaction/span tracking
- Comprehensive test coverage (500+ lines)
- 7 example programs demonstrating all features

## Documentation
- Complete integration guide in docs/sinks.md
- Quick reference examples in docs/quick-reference
- README updates with Sentry examples
- Inline documentation for all public APIs

Fixes #46

Author: willibrandon

.github/workflows/ci.yml

@@ -64,6 +64,12 @@ jobs:
         go test -v ./...
         go test -race -v ./...
 
+    - name: Test Sentry adapter module
+      run: |
+        cd adapters/sentry
+        go test -v ./...
+        go test -race -v ./...
+
     - name: Test coverage
       run: go test -coverprofile=coverage.out ./...
       if: matrix.os == 'ubuntu-latest' && matrix.go == '1.23'

.gitignore

@@ -81,3 +81,12 @@ cmd/mtlog-lsp/mtlog-lsp
 zed-extension/mtlog/target/
 zed-extension/mtlog/extension.wasm
 zed-extension/mtlog/Cargo.lock
+
+# Sentry test infrastructure (generated files)
+docker/sentry-config/
+docker/relay/
+docker/nginx/
+docker/postgres/
+docker/.env
+docker/docker-compose.test.yml
+docker/docker-compose.test.yml.new

README.md

@@ -43,6 +43,7 @@ mtlog is a high-performance structured logging library for Go, inspired by [Seri
 - **Elasticsearch sink** for centralized log storage and search
 - **Splunk sink** with HEC (HTTP Event Collector) support
 - **OpenTelemetry (OTLP) sink** with gRPC/HTTP transport, batching, and trace correlation
+- **Sentry integration** with error tracking, performance monitoring, and intelligent sampling
 - **Conditional sink** for predicate-based routing with zero overhead
 - **Router sink** for multi-destination routing with FirstMatch/AllMatch modes
 - **Async sink wrapper** for high-throughput scenarios
@@ -727,6 +728,37 @@ mtlog.WithSplunkAdvanced("http://localhost:8088",
 )
 ```
 
+### Sentry Integration
+
+```go
+import (
+    "github.com/willibrandon/mtlog"
+    "github.com/willibrandon/mtlog/adapters/sentry"
+)
+
+// Basic Sentry error tracking
+sink, _ := sentry.WithSentry("https://key@sentry.io/project")
+log := mtlog.New(mtlog.WithSink(sink))
+
+// With sampling for high-volume applications
+sink, _ := sentry.WithSentry("https://key@sentry.io/project",
+    sentry.WithFixedSampling(0.1), // 10% sampling
+)
+log := mtlog.New(mtlog.WithSink(sink))
+
+// Advanced configuration with performance monitoring
+sink, _ := sentry.WithSentry("https://key@sentry.io/project",
+    sentry.WithEnvironment("production"),
+    sentry.WithRelease("v1.2.3"),
+    sentry.WithTracesSampleRate(0.2),
+    sentry.WithProfilesSampleRate(0.1),
+    sentry.WithAdaptiveSampling(0.01, 0.5), // 1% to 50% adaptive
+    sentry.WithRetryPolicy(3, time.Second),
+    sentry.WithStackTraceCache(1000),
+)
+log := mtlog.New(mtlog.WithSink(sink))
+```
+
 ### Async and Durable Sinks
 
 ```go
@@ -963,7 +995,7 @@ Benchmark results on AMD Ryzen 9 9950X:
 
 ## Examples
 
-See the [examples](./examples) directory and [OTEL examples](./adapters/otel/examples) for complete examples:
+See the [examples](./examples) directory and adapter examples ([OTEL](./adapters/otel/examples), [Sentry](./adapters/sentry/examples), [middleware](./adapters/middleware/examples)) for complete examples:
 
 - [Basic logging](./examples/basic/main.go)
 - [Using enrichers](./examples/enrichers/main.go)
@@ -992,6 +1024,13 @@ See the [examples](./examples) directory and [OTEL examples](./adapters/otel/exa
 - [OTEL with metrics](./adapters/otel/examples/metrics/main.go)
 - [OTEL with sampling](./adapters/otel/examples/sampling/main.go)
 - [OTEL with TLS](./adapters/otel/examples/tls/main.go)
+- [Sentry error tracking](./adapters/sentry/examples/basic/main.go)
+- [Sentry with context](./adapters/sentry/examples/context/main.go)
+- [Sentry breadcrumbs](./adapters/sentry/examples/breadcrumbs/main.go)
+- [Sentry with retry](./adapters/sentry/examples/retry/main.go)
+- [Sentry performance monitoring](./adapters/sentry/examples/performance/main.go)
+- [Sentry metrics dashboard](./adapters/sentry/examples/metrics/main.go)
+- [Sentry sampling strategies](./adapters/sentry/examples/sampling/main.go)
 - [Async logging](./examples/async/main.go)
 - [Durable buffering](./examples/durable/main.go)
 - [Dynamic levels](./examples/dynamic-levels/main.go)