docs: document TBS config options for standalone apm server (#10740)

* docs: document TBS config options for standalone apm server

* docs: add tbs option documentation

* docs: fix spacing

apm-server.docker.yml

@@ -279,6 +279,18 @@ apm-server:
     # Configure curve types for ECDHE based cipher suites.
     #ssl.curve_types: []
 
+  #---------------------------- APM Server - tail-based sampling ----------------------------
+
+  #sampling.tail:
+    # Set to `true` to enable tail based sampling. Disabled by default.
+    #enabled: false
+
+    # Synchronization interval for multiple APM Servers. Should be in the order of tens of seconds or low minutes.
+    #interval: 1m
+
+    # Criteria used to match a root transaction to a sample rate.
+    #policies: []
+
 # Sets the maximum number of CPUs that can be executing simultaneously. The
 # default is the number of logical CPUs available in the system.
 #max_procs:

apm-server.yml

@@ -279,6 +279,18 @@ apm-server:
     # Configure curve types for ECDHE based cipher suites.
     #ssl.curve_types: []
 
+  #---------------------------- APM Server - tail-based sampling ----------------------------
+
+  #sampling.tail:
+    # Set to `true` to enable tail based sampling. Disabled by default.
+    #enabled: false
+
+    # Synchronization interval for multiple APM Servers. Should be in the order of tens of seconds or low minutes.
+    #interval: 1m
+
+    # Criteria used to match a root transaction to a sample rate.
+    #policies: []
+
 # Sets the maximum number of CPUs that can be executing simultaneously. The
 # default is the number of logical CPUs available in the system.
 #max_procs:

docs/legacy/configuration-process.asciidoc

@@ -139,6 +139,27 @@ Defaults to `debug/vars`.
 Enables self instrumentation of the APM Server itself.
 Disabled by default.
 
+[float]
+[[configuration-tbs]]
+=== Configuration options: tail-based sampling
+
+[[sampling.tail.enabled]]
+[float]
+==== `sampling.tail.enabled`
+Set to `true` to enable tail based sampling.
+Disabled by default.
+
+[[sampling.tail.interval]]
+[float]
+==== `sampling.tail.interval`
+Synchronization interval for multiple APM Servers.
+Should be in the order of tens of seconds or low minutes.
+
+[[sampling.tail.policies]]
+[float]
+==== `sampling.tail.policies`
+Criteria used to match a root transaction to a sample rate.
+
 [float]
 === Configuration options: `max_procs`