[APM] Document apm_oss.metricsIndices and `apm_oss.sourcemap… (#50312)

Closes #49847.

docs/settings/apm-settings.asciidoc

@@ -23,8 +23,6 @@ xpack.apm.ui.transactionGroupBucketSize:: Number of top transaction groups displ
 
 xpack.apm.ui.maxTraceItems:: Max number of child items displayed when viewing trace details. Defaults to `1000`.
 
-apm_oss.apmAgentConfigurationIndex:: Index containing agent configuration settings. Defaults to `.apm-agent-configuration`.
-
 apm_oss.indexPattern:: Index pattern is used for integrations with Machine Learning and Kuery Bar. It must match all apm indices. Defaults to `apm-*`.
 
 apm_oss.errorIndices:: Matcher for indices containing error documents. Defaults to `apm-*`.
@@ -34,3 +32,8 @@ apm_oss.onboardingIndices:: Matcher for indices containing onboarding documents.
 apm_oss.spanIndices:: Matcher for indices containing span documents. Defaults to `apm-*`.
 
 apm_oss.transactionIndices:: Matcher for indices containing transaction documents. Defaults to `apm-*`.
+
+apm_oss.metricsIndices:: Matcher for indices containing metric documents. Defaults to `apm-*`.
+
+apm_oss.sourcemapIndices:: Matcher for indices containing sourcemap documents. Defaults to `apm-*`.
+