[Uptime] Fix nesting of x509 subject (elastic#18139)

In elastic#17687 we added additional x509 fields. The mapping accidentally
double nested subject fields as subject.subject. This fixes that.

There are no tests here because we don't really have testing around
mappings, and it's sort of difficult to test without being repetitive,
so none are included here.

(cherry picked from commit a81bbda)

heartbeat/docs/fields.asciidoc

@@ -8020,7 +8020,7 @@ example: SHA256-RSA
 --
 
 
-*`tls.server.x509.subject.subject.common_name`*::
+*`tls.server.x509.subject.common_name`*::
 +
 --
 List of common names (CN) of subject.
@@ -8031,14 +8031,14 @@ example: r2.shared.global.fastly.net
 
 --
 
-*`tls.server.x509.subject.subject.common_name.text`*::
+*`tls.server.x509.subject.common_name.text`*::
 +
 --
 type: text
 
 --
 
-*`tls.server.x509.subject.subject.distinguished_name`*::
+*`tls.server.x509.subject.distinguished_name`*::
 +
 --
 Distinguished name (DN) of the certificate subject entity.

heartbeat/monitors/active/dialchain/_meta/fields.yml

@@ -140,7 +140,7 @@
               - name: subject
                 type: group
                 fields:
-                - name: subject.common_name
+                - name: common_name
                   type: keyword
                   ignore_above: 1024
                   description: List of common names (CN) of subject.
@@ -150,7 +150,7 @@
                     - name: text
                       type: text
                       analyzer: simple
-                - name: subject.distinguished_name
+                - name: distinguished_name
                   type: keyword
                   ignore_above: 1024
                   description: Distinguished name (DN) of the certificate subject entity.