Document the reason for skipped APM tests (elastic#201214)

Closes elastic#199843

### Summary

This PR documents the reason for skipped Indices diagnostics API tests
to avoid confusion in the future. APM diagnostics is an experimental
internal tool. Indices diagnostics determine whether ingest pipelines
were installed correctly by verifying the presence of the
`observer.version` field in grok processor, this approach isn't reliable
anymore. We should consider implementing improvement or sunsetting the
feature if there is no maintainer.

x-pack/test/api_integration/deployment_agnostic/apis/observability/apm/diagnostics/indices.spec.ts

@@ -21,9 +21,10 @@ export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderCon
 
   let apmSynthtraceEsClient: ApmSynthtraceEsClient;
 
-  describe('Diagnostics: Indices', () => {
-    describe.skip('When there is no data', () => {
-      it('returns empty response`', async () => {
+  // Failing tests were skipped because the current solution for verifying ingest pipelines needs improvement
+  describe.skip('Diagnostics: Indices', () => {
+    describe('When there is no data', () => {
+      it('returns empty response', async () => {
         const { status, body } = await apmApiClient.adminUser({
           endpoint: 'GET /internal/apm/diagnostics',
         });
@@ -34,7 +35,7 @@ export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderCon
       });
     });
 
-    describe.skip('When data is ingested', () => {
+    describe('When data is ingested', () => {
       before(async () => {
         const instance = apm
           .service({ name: 'synth-go', environment: 'production', agentName: 'go' })
@@ -68,7 +69,7 @@ export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderCon
       });
     });
 
-    describe.skip('When data is ingested without the necessary index templates', () => {
+    describe('When data is ingested without the necessary index templates', () => {
       before(async () => {
         await es.indices.deleteDataStream({ name: 'traces-apm-*' });
         await es.indices.deleteIndexTemplate({ name: ['traces-apm'] });
@@ -114,7 +115,7 @@ export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderCon
       });
     });
 
-    describe.skip('ingest pipelines', () => {
+    describe('ingest pipelines', () => {
       before(async () => {
         const instance = apm
           .service({ name: 'synth-go', environment: 'production', agentName: 'go' })
@@ -138,7 +139,7 @@ export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderCon
         await apmSynthtraceEsClient.clean();
       });
 
-      describe.skip('an ingest pipeline is removed', () => {
+      describe('an ingest pipeline is removed', () => {
         before(async () => {
           const datastreamToUpdate = await es.indices.getDataStream({
             name: 'metrics-apm.internal-default',
@@ -168,7 +169,7 @@ export default function ApiTest({ getService }: DeploymentAgnosticFtrProviderCon
         });
       });
 
-      describe.skip('an ingest pipeline is changed', () => {
+      describe('an ingest pipeline is changed', () => {
         before(async () => {
           const datastreamToUpdate = await es.indices.getDataStream({
             name: 'metrics-apm.internal-default',