Improve tracing

docs/content/observability/tracing/jaeger.md

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+# Jaeger
+
+To enable the Jaeger:
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger
+```
+
+!!! warning
+    Traefik is only able to send data over the compact thrift protocol to the [Jaeger agent](https://www.jaegertracing.io/docs/deployment/#agent).
+
+#### `samplingServerURL`
+
+_Required, Default="http://localhost:5778/sampling"_
+
+Sampling Server URL is the address of jaeger-agent's HTTP sampling server.
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    samplingServerURL = "http://localhost:5778/sampling"
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.samplingServerURL="http://localhost:5778/sampling"
+```
+
+#### `samplingType`
+
+_Required, Default="const"_
+
+Sampling Type specifies the type of the sampler: `const`, `probabilistic`, `rateLimiting`.
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    samplingType = "const"
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.samplingType="const"
+```
+
+#### `samplingParam`
+
+_Required, Default=1.0_
+
+Sampling Param is a value passed to the sampler.
+
+Valid values for Param field are:
+
+- for `const` sampler, 0 or 1 for always false/true respectively
+- for `probabilistic` sampler, a probability between 0 and 1
+- for `rateLimiting` sampler, the number of spans per second
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    samplingParam = 1.0
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.samplingParam="1.0"
+```
+
+#### `localAgentHostPort`
+
+_Required, Default="127.0.0.1:6831"_
+
+Local Agent Host Port instructs reporter to send spans to jaeger-agent at this address.
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    localAgentHostPort = "127.0.0.1:6831"
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.localAgentHostPort="127.0.0.1:6831"
+```
+
+#### `gen128Bit`
+
+_Optional, Default=false_
+
+Generate 128-bit trace IDs, compatible with OpenCensus.
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    gen128Bit = true
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.gen128Bit
+```
+
+#### `propagation`
+
+_Required, Default="jaeger"_
+
+Set the propagation header type.
+This can be either:
+
+- `jaeger`, jaeger's default trace header.
+- `b3`, compatible with OpenZipkin
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    propagation = "jaeger"
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.propagation="jaeger"
+```
+
+#### `traceContextHeaderName`
+
+_Required, Default="uber-trace-id"_
+
+Trace Context Header Name is the http header name used to propagate tracing context.
+This must be in lower-case to avoid mismatches when decoding incoming headers.
+
+```toml tab="File"
+[tracing]
+  [tracing.jaeger]
+    traceContextHeaderName = "uber-trace-id"
+```
+
+```bash tab="CLI"
+--tracing
+--tracing.jaeger.traceContextHeaderName="uber-trace-id"
+```