Add sampling

Author: cleptric

develop-docs/sdk/telemetry/spans/span-sampling.mdx

@@ -4,17 +4,99 @@ title: Span Sampling & Filtering
 
 Any APIs exposed to the user to sample or filter spans must adhere to the following design principles:
 
-- The APIs are optimized for trace completness
+- The APIs are optimized for trace completeness
 - The APIs are optimized for conclusive sampling decisions
 
 ## Sample root spans with `tracesSampleRate` & `tracesSampler`
 
+The SDK is automatically initialized with a `tracesSampleRate` of `0`.
+When starting a root span, the configured rate is compared against a random number between 0 and 1 to decide if this root span will be sampled or not.
 
+If the SDK is configured with a `tracesSampler`, the `tracesSampleRate` no longer applies.
+The `tracesSampler` callback must receive sufficient arguments from users to define their own sampling rules.
+This can include but is not limited to certain attributes from the root span, such as HTTP headers.
+The return value of the `tracesSampler` is a float between `0.0` and `1.0`.
+
+```js
+Sentry.init({
+  tracesSampler: ({ name, attributes, parentSampleRate }) => {
+    // Inherit the trace parent's sample rate if there is one. Sampling is deterministic 
+    // for one trace, e.g. if the parent was sampled, all children will be sampled at the same rate.
+    if (typeof parentSampleRate === "number") {
+      return parentSampleRate;
+    }
+    
+    // Else, use a default sample rate (replacing tracesSampleRate).
+    return 0.5;
+  }
+})
+```
+
+The `parentSampleRate` is a propagated value inside the baggage, using key `sentry-sample_rand`.
+The value stems from a truly random number between 0 and 1, generated when a new trace is started. If the SDK does not receive such a number in an incoming trace, a new, truly random number between 0 and 1 is generated.
+
+In the following cases, the SDK must compare sample rates against this `parentSampleRate` instead of `math.random()`:
+  
+  - When a `tracesSampler` is configured, e.g. `trace["sentry-sample_rand"] < tracesSampler()`
+  
+  - When the SDK is the head of trace, this applies to sample decisions based on `tracesSampleRate`, e.g. `trace['sentry-sample_rand'] < config.tracesSampleRate`
+
+If the `sentry-sample_rate` (`parentSampleRate`) is not available for any reason for an inbound trace, but the trace has the sampled flag set to true, the SDK injects `parentSampleRate: 1.0` into the `tracesSampler`.
+
+If no `tracesSampler` is configured, a propagated sampling decision via the traceparent takes precedence over the `tracesSampleRate`. This behavior can be disabled by defining a `tracesSampler`.
+ 
+### Parent Sampling Origins
+
+If the SDK can parse an org ID from the configured DSN, this value must be propagated as a baggage entry with the key `sentry-org`. Given a DSN of `https://[email protected]/1`, the org ID is 1, based on `o1`.
+
+On incoming traces, the SDK must compare the `sentry-org` baggage value against its own parsed value from the DSN. Only if both match, the parent sampling decisions applies.
+
+This behavior can be disabled by setting `strictTracePropagation: false` in the SDK init call.
+
+The SDK must be configurable with an optional `org: <org-id>` setting that takes precedence over the parsed value from the DSN.
 
 ## Filter spans with `ignoreSpans` & integration config
 
+The SDK must implement a mechanism for users to filter out spans. The result must be binary (true/false).
+The `ignoreSpans` option accepts a glob pattern or string.
+The `integrations` option can perform in similar fashion or make explicit opt-out possible via a bool flag.
 
+If both options are not feasible to be implemented in certain SDKs, other approaches must be explored that have the same outcome.
+
+```js
+Sentry.init({
+  ignoreSpans: [
+		'GET /about',
+		'events.signal *',
+	],
+	integrations: [
+		fsIntegration: {
+			ignoreSpans: [
+				'fs.read',
+			],
+			readSpans: true,
+			writeSpans: false,
+		}
+	]
+})
+```
 
 ## Sanitize span attributes with `beforeSendSpans`
 
+This callback must not allow the removal of any spans from the span tree.
+It receives a deep copy of all spans in the span tree and their attributes.
+
+```
+[
+  {
+    'name': 'GET /',
+    'attributes': [
+      'http.request.method': 'GET',
+      'http.response.status_code': 200,
+    ]
+  },
+]
+```
 
+Users can mutate any exposed properties to perform sanitation on sensitive data or Pii.
+The return value `beforeSendSpans` should be merged with the original span tree prior to emission.