[APM] Make fleet optional again: Store source maps in APM index

[sorenlouv]

Pitch: https://github.com/elastic/apm-dev/issues/852 (internal)

APM currently requires Fleet for powering several APM-specific features. To make Fleet optional we need to replicate those features.

Source maps

Source maps are currently stored as Fleet artifacts. In addition to this we should store source maps in an APM owned index that APM Server can query directly.

Privileges
The source map index should most likely be accessed using the internal kibana user. The reason for this is that we need to create an API token for APM Server with access to the index. We may also need to run the migrations in the background (with the system user)

The kibana system user may not have access to the source map index by default. Privileges for built in roles is defined her:

https://github.com/elastic/elasticsearch/blob/fb38095598953f1e5eb3ade7272f2c4e1064f7ed/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java

Privilieges added to kibana_system in elastic/elasticsearch#92160.

Deprecate existing source map settings
There is a legacy setting xpack.apm.indices.sourcemap that is not longer used. This should be marked as deprecated (and later removed) to avoid confusion.

kibana/x-pack/plugins/apm/server/index.ts

Line 54 in 185993c

sourcemap: schema.string({ defaultValue: 'apm-*' }),

Index naming
Possible name for the index: .apm-source-map to align with other APM system indices:

kibana/x-pack/plugins/apm/server/routes/settings/apm_indices/get_apm_indices.ts

Lines 53 to 56 in b9ffc29

	// system indices, not configurable
	apmAgentConfigurationIndex: '.apm-agent-configuration',
	apmCustomLinkIndex: '.apm-custom-link',
	};

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)

[sorenlouv]

Upload source maps

for run in {1..1000}; do
  curl --request POST \
    --url https://deploy-kibana-mjgyh-custom.kb.us-west2.gcp.elastic-cloud.com/api/apm/sourcemaps \
    --header 'Authorization: ApiKey encodeApiKey' \
    --header 'Content-Type: multipart/form-data' \
    --header 'kbn-xsrf: true' \
    --form service_version=4.20.$run \
    --form service_name=opbeans-node \
    --form bundle_filepath=https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.1/jquery.min.js \
    --form sourcemap=@/Users/sqren/Downloads/source-map-files/jquery.min.map
done

Verify source maps show up in new index:

GET .apm-source-map/_search