Document operations guidance in README

README.md

@@ -5,7 +5,8 @@ Minimal AWS Lambda starter with **structured JSON logging**, a small **metrics h
 ## Features
 
 - Structured logger that emits one JSON object per line (CloudWatch-friendly)
-- Simple EMF-style metrics buffer you can flush as a log line
+- Correlation IDs for tracing a single invocation across log lines
+- Simple EMF-style metrics buffer you can flush as a log line (counters, gauges, timings)
 - Example Lambda handler wiring logger + metrics
 - Pure unit tests (no network, no AWS credentials)
 
@@ -23,21 +24,61 @@ pytest -q
 ```python
 from lambda_obs.handler import handler
 
-result = handler({"name": "world"}, None)
-print(result)  # {"message": "hello world", "ok": true}
+result = handler({"name": "world", "correlation_id": "demo-1"}, None)
+print(result)  # {"message": "hello world", "ok": true, "correlation_id": "demo-1"}
 ```
 
+Pass a correlation id via event field, `headers.x-correlation-id`, or Lambda `context.aws_request_id`.
+
 ## Layout
 
 ```
 src/lambda_obs/
-  logging.py   # structured JSON logger
-  metrics.py   # metrics helper
+  logging.py   # structured JSON logger + correlation ids
+  metrics.py   # metrics helper (count / set / timing)
   handler.py   # example Lambda entrypoint
 tests/
 .github/workflows/ci.yml
 ```
 
+## Operations
+
+### Structured logs in CloudWatch
+
+Each log line is a single JSON object. Filter and explore with CloudWatch Logs Insights, for example:
+
+```
+fields @timestamp, level, message, correlation_id
+| filter ispresent(correlation_id)
+| sort @timestamp desc
+| limit 50
+```
+
+### Metrics (EMF)
+
+`Metrics.flush()` writes an Embedded Metric Format line to stdout. In Lambda, CloudWatch extracts the namespace, dimensions, and metric values automatically—no extra agent required.
+
+Useful starter metrics from the example handler:
+
+- `Invocations` / `Success` / `Errors` (Count)
+- `HandlerDuration` (Milliseconds)
+
+### Correlation
+
+Prefer one correlation id per request. Propagate it to downstream calls and keep it on every log line via `Logger.with_correlation_id(...)`.
+
+### Local vs deployed
+
+| Concern | Local | Deployed Lambda |
+| --- | --- | --- |
+| Credentials | Not required for unit tests | Execution role only |
+| Logs | stdout / captured in tests | CloudWatch Logs |
+| Metrics | JSON line on stdout | EMF → CloudWatch Metrics |
+
+### CI
+
+GitHub Actions runs `pytest` across Python 3.10–3.12 on pushes and pull requests to `main` and `develop`.
+
 ## Design notes
 
 Inspired by the *ideas* behind AWS Lambda Powertools (structured logs + custom metrics), but this is an original, tiny teaching scaffold — not a fork or copy of AWS Powertools code.