Improve tracing

docs/content/observability/tracing/overview.md

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+# Tracing
+
+Visualize the Requests Flow
+{: .subtitle }
+
+The tracing system allows developers to visualize call flows in their infrastructure.
+
+Traefik uses OpenTracing, an open standard designed for distributed tracing.
+
+Traefik supports five tracing backends:
+
+- [Jaeger](./jaeger.md)
+- [Zipkin](./zipkin.md)
+- [DataDog](./datadog.md)
+- [Instana](./instana.md)
+- [Haystack](./haystack.md)
+
+## Configuration
+
+By default, Traefik uses Jaeger as tracing backend.
+
+To enable the tracing:
+
+```toml tab="File"
+[tracing]
+```
+
+```bash tab="CLI"
+--tracing
+```
+
+### Common Options
+
+#### `serviceName`
+
+_Required, Default="traefik"_
+
+Service name used in selected backend.
+
+```toml tab="File"
+[tracing]
+  serviceName = "traefik"
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.serviceName="traefik"
+```
+
+#### `spanNameLimit`
+
+_Required, Default=0_
+
+Span name limit allows for name truncation in case of very long names.
+This can prevent certain tracing providers to drop traces that exceed their length limits.
+
+`0` means no truncation will occur.
+
+```toml tab="File"
+[tracing]
+  spanNameLimit = 150
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.spanNameLimit=150
+```