apm: Set log.level: error on all apm error logs

This commit sets `log.level: "error"` as a `constant_keyword` for all
apm error logs.

Part of elastic/apm-server#13956

Signed-off-by: Marc Lopez Rubio <[email protected]>

x-pack/plugin/apm-data/src/main/resources/index-templates/logs-apm.error@template.yaml

@@ -26,6 +26,9 @@ template:
       processor.event:
         type: constant_keyword
         value: error
+      log.level:
+        type: constant_keyword
+        value: error
   settings:
     index:
       default_pipeline: logs-apm.error@default-pipeline

x-pack/plugin/apm-data/src/main/resources/resources.yaml

@@ -1,7 +1,7 @@
 # "version" holds the version of the templates and ingest pipelines installed
 # by xpack-plugin apm-data. This must be increased whenever an existing template or
 # pipeline is changed, in order for it to be updated on Elasticsearch upgrade.
-version: 8
+version: 9
 
 component-templates:
   # Data lifecycle.

x-pack/plugin/apm-data/src/yamlRestTest/resources/rest-api-spec/test/20_error_logs.yml

@@ -0,0 +1,26 @@
+---
+setup:
+  - do:
+      cluster.health:
+        wait_for_events: languid
+
+---
+"Test logs-apm.error-* error log fields":
+  - do:
+      bulk:
+        index: logs-apm.error-log-level-testing
+        refresh: true
+        body:
+          # Non-empty error.exception.message used
+          - create: {}
+          - '{"@timestamp": "2017-06-22", "error": {"log": {"message": "loglevel"}, "exception": [{"message": "exception_used"}]}}'
+
+  - is_false: errors
+
+  - do:
+      search:
+        index: logs-apm.error-log-level-testing
+        body:
+          fields: ["log.level"]
+  - length: { hits.hits: 1 }
+  - match: { hits.hits.0.fields: {"log.level": ["error"]} }