Add CLAUDE.md documentation

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+This is the PostgreSQL Server Exporter for Prometheus, written in Go. It exports PostgreSQL metrics for monitoring with Prometheus. The project supports PostgreSQL versions 11-16 and includes multi-target functionality for monitoring multiple PostgreSQL instances from a single exporter instance.
+
+## Build and Development Commands
+
+### Building
+- `make build` - Build the binary using promu
+- `make promu` - Install the promu build tool
+- `make docker` - Build Docker image (requires promu crossbuild first)
+
+### Testing
+- `make test` - Run all tests
+- `make test-short` - Run short tests only
+- Integration tests: `DATA_SOURCE_NAME='postgresql://postgres:test@localhost:5432/circle_test?sslmode=disable' GOOPTS='-v -tags integration' make test`
+
+### Code Quality
+- `make lint` - Run golangci-lint
+- `make style` - Check code formatting with gofmt
+- `make format` - Format code with go fmt
+- `make vet` - Run go vet
+- `make check_license` - Verify license headers
+- `make yamllint` - Check YAML files
+
+### Dependencies
+- `make deps` - Download dependencies
+- `make unused` - Check for unused dependencies
+- `update-go-deps` - Update Go dependencies
+
+## Architecture
+
+### Core Components
+
+**Main Entry Point**: `cmd/postgres_exporter/main.go`
+- Uses kingpin for CLI argument parsing
+- Supports both single-target and multi-target modes
+- Configuration via flags, environment variables, or YAML config file
+
+**Collector Framework**: `collector/` package
+- `collector.go` - Base collector interface and factory system
+- Each collector handles specific PostgreSQL metrics (e.g., `pg_stat_database`, `pg_locks`)
+- Collectors can be enabled/disabled via command-line flags
+- Uses Prometheus client library for metric definitions
+
+**Configuration**: `config/` package
+- `config.go` - Main configuration structure
+- `dsn.go` - Data Source Name handling for PostgreSQL connections
+- Supports authentication modules for multi-target setups
+
+### Collector Types
+
+Individual collectors in `collector/` directory handle different PostgreSQL statistics:
+- `pg_database.go` - Database-level metrics
+- `pg_stat_database.go` - Database statistics
+- `pg_stat_bgwriter.go` - Background writer stats
+- `pg_locks.go` - Lock information
+- `pg_replication.go` - Replication metrics
+- And many others for specific PostgreSQL features
+
+### Multi-Target Support
+
+The exporter supports monitoring multiple PostgreSQL instances via the `/probe` endpoint with target parameter. Authentication modules in the config file enable secure credential management without exposing passwords in URLs.
+
+## Key Configuration
+
+- Default config file: `postgres_exporter.yml`
+- Primary environment variable: `DATA_SOURCE_NAME` for PostgreSQL connection
+- Default metrics endpoint: `:9187/metrics`
+- Custom queries can be defined in `queries.yaml` (deprecated feature)
+
+## Testing Setup
+
+For integration tests, you need a running PostgreSQL instance:
+```bash
+docker run -p 5432:5432 -e POSTGRES_DB=circle_test -e POSTGRES_USER=postgres -e POSTGRES_PASSWORD=test -d postgres
+```
+
+## Development Notes
+
+- The project uses Go modules (`go.mod`)
+- Follows Prometheus exporter conventions
+- All collectors implement the `Collector` interface
+- Metric names use the `pg_` prefix by default
+- Code must include Apache 2.0 license headers
+- Uses the prometheus-community Docker repository for images