Add OpenTelemetry tracing and metrics support

docs/content/observability/tracing/opentelemetry.md

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+---
+title: "Traefik OpenTelemetry Documentation"
+description: "Traefik supports several tracing backends, including OpenTelemetry. Learn how to implement it for observability in Traefik Proxy. Read the technical documentation."
+---
+
+# OpenTelemetry
+
+To enable the OpenTelemetry tracer:
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry: {}
+```
+
+```toml tab="File (TOML)"
+[tracing]
+  [tracing.openTelemetry]
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry=true
+```
+
+!!! info "The OpenTelemetry trace reporter will export traces to the collector using HTTP by default, see the [gRPC Section](#grpc-configuration) to use gRPC."
+
+!!! info "Trace sampling"
+
+	By default, the OpenTelemetry trace reporter will sample 100% of traces.
+	See [OpenTelemetry's SDK configuration](https://opentelemetry.io/docs/reference/specification/sdk-environment-variables/#general-sdk-configuration) to customize the sampling strategy.
+
+#### `address`
+
+_Required, Default="localhost:4318", Format="`<host>:<port>`"_
+
+Address of the OpenTelemetry Collector to send spans to.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    address: localhost:4318
+```
+
+```toml tab="File (TOML)"
+[tracing]
+  [tracing.openTelemetry]
+    address = "localhost:4318"
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.address=localhost:4318
+```
+
+#### `headers`
+
+_Optional, Default={}_
+
+Additional headers sent with spans by the reporter to the OpenTelemetry Collector.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    headers:
+      foo: bar
+      baz: buz
+```
+
+```toml tab="File (TOML)"
+[tracing]
+  [tracing.openTelemetry.headers]
+    foo = "bar"
+    baz = "buz"
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.headers.foo=bar --tracing.openTelemetry.headers.baz=buz
+```
+
+#### `insecure`
+
+_Optional, Default=false_
+
+Allows reporter to send spans to the OpenTelemetry Collector without using a secured protocol.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    insecure: true
+```
+
+```toml tab="File (TOML)"
+[tracing]
+  [tracing.openTelemetry]
+    insecure = true
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.insecure=true
+```
+
+#### `path`
+
+_Required, Default="/v1/traces"_
+
+Allows to override the default URL path used for sending traces.
+This option has no effect when using gRPC transport.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    path: /foo/v1/traces
+```
+
+```toml tab="File (TOML)"
+[tracing]
+  [tracing.openTelemetry]
+    path = "/foo/v1/traces"
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.path=/foo/v1/traces
+```
+
+#### `tls`
+
+_Optional_
+
+Defines the TLS configuration used by the reporter to send spans to the OpenTelemetry Collector.
+
+##### `ca`
+
+_Optional_
+
+`ca` is the path to the certificate authority used for the secure connection to the OpenTelemetry Collector,
+it defaults to the system bundle.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    tls:
+      ca: path/to/ca.crt
+```
+
+```toml tab="File (TOML)"
+[tracing.openTelemetry.tls]
+  ca = "path/to/ca.crt"
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.tls.ca=path/to/ca.crt
+```
+
+##### `cert`
+
+_Optional_
+
+`cert` is the path to the public certificate used for the secure connection to the OpenTelemetry Collector.
+When using this option, setting the `key` option is required.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    tls:
+      cert: path/to/foo.cert
+      key: path/to/foo.key
+```
+
+```toml tab="File (TOML)"
+[tracing.openTelemetry.tls]
+  cert = "path/to/foo.cert"
+  key = "path/to/foo.key"
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.tls.cert=path/to/foo.cert
+--tracing.openTelemetry.tls.key=path/to/foo.key
+```
+
+##### `key`
+
+_Optional_
+
+`key` is the path to the private key used for the secure connection to the OpenTelemetry Collector.
+When using this option, setting the `cert` option is required.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    tls:
+      cert: path/to/foo.cert
+      key: path/to/foo.key
+```
+
+```toml tab="File (TOML)"
+[tracing.openTelemetry.tls]
+  cert = "path/to/foo.cert"
+  key = "path/to/foo.key"
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.tls.cert=path/to/foo.cert
+--tracing.openTelemetry.tls.key=path/to/foo.key
+```
+
+##### `insecureSkipVerify`
+
+_Optional, Default=false_
+
+If `insecureSkipVerify` is `true`,
+the TLS connection to the OpenTelemetry Collector accepts any certificate presented by the server regardless of the hostnames it covers.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    tls:
+      insecureSkipVerify: true
+```
+
+```toml tab="File (TOML)"
+[tracing.openTelemetry.tls]
+  insecureSkipVerify = true
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.tls.insecureSkipVerify=true
+```
+
+#### gRPC configuration
+
+_Optional_
+
+This instructs the reporter to send spans to the OpenTelemetry Collector using gRPC.
+
+```yaml tab="File (YAML)"
+tracing:
+  openTelemetry:
+    grpc: {}
+```
+
+```toml tab="File (TOML)"
+[tracing]
+  [tracing.openTelemetry.grpc]
+```
+
+```bash tab="CLI"
+--tracing.openTelemetry.grpc=true
+```