Add sampling doc for JS (#2482)

Co-authored-by: Patrice Chalin <chalin@users.noreply.github.com>
Co-authored-by: Severin Neumann <neumanns@cisco.com>
Co-authored-by: Marc Pichler <marcpi@edu.aau.at>

content/en/docs/instrumentation/js/sampling.md

@@ -0,0 +1,94 @@
+---
+title: Sampling
+weight: 9
+---
+
+[Sampling](/docs/concepts/sampling/) is a process that restricts the amount of
+traces that are generated by a system. The JavaScript SDK offers several
+[head samplers](/docs/concepts/sampling#head-sampling).
+
+## Default behavior
+
+By default, all spans are sampled, and thus, 100% of traces are sampled. If you
+do not need to manage data volume, don't bother setting a sampler.
+
+## TraceIDRatioBasedSampler
+
+When sampling, the most common head sampler to use is the
+TraceIdRatioBasedSampler. It deterministically samples a percentage of traces
+that you pass in as a parameter.
+
+### Environment Variables
+
+You can configure the TraceIdRatioBasedSampler with environment variables:
+
+```shell
+export OTEL_TRACES_SAMPLER="traceidratio"
+export OTEL_TRACES_SAMPLER_ARG="0.1"
+```
+
+This tells the SDK to sample spans such that only 10% of traces get created.
+
+### Node.js
+
+You can also configure the TraceIdRatioBasedSampler in code. Here's an example
+for Node.js:
+
+<!-- prettier-ignore-start -->
+{{< tabpane lang=shell persistLang=false >}}
+
+{{< tab TypeScript >}}
+import { TraceIdRatioBasedSampler } from '@opentelemetry/sdk-trace-node';
+
+const samplePercentage = 0.1;
+
+const sdk = new NodeSDK({
+  // Other SDK configuration parameters go here
+  sampler: new TraceIdRatioBasedSampler(samplePercentage),
+});
+{{< /tab >}}
+
+{{< tab JavaScript >}}
+const { TraceIdRatioBasedSampler } = require('@opentelemetry/sdk-trace-node');
+
+const samplePercentage = 0.1;
+
+const sdk = new NodeSDK({
+  // Other SDK configuration parameters go here
+  sampler: new TraceIdRatioBasedSampler(samplePercentage),
+});
+{{< /tab >}}
+
+{{< /tabpane >}}
+<!-- prettier-ignore-end -->
+
+### Browser
+
+You can also configure the TraceIdRatioBasedSampler in code. Here's an example
+for browser apps:
+
+<!-- prettier-ignore-start -->
+{{< tabpane lang=shell persistLang=false >}}
+
+{{< tab TypeScript >}}
+import { WebTracerProvider, TraceIdRatioBasedSampler } from '@opentelemetry/sdk-trace-web';
+
+const samplePercentage = 0.1;
+
+const provider = new WebTracerProvider({
+    sampler: new TraceIdRatioBasedSampler(samplePercentage),
+});
+{{< /tab >}}
+
+{{< tab JavaScript >}}
+const { WebTracerProvider, TraceIdRatioBasedSampler } = require('@opentelemetry/sdk-trace-web');
+
+const samplePercentage = 0.1;
+
+const provider = new WebTracerProvider({
+    sampler: new TraceIdRatioBasedSampler(samplePercentage),
+});
+{{< /tab >}}
+
+{{< /tabpane >}}
+<!-- prettier-ignore-end -->