Apply task manager backpressure whenever a 500 error is returned in t…

…he task store (elastic#198418)

In this PR, I'm making the task manager apply backpressure whenever a
500 error is returned in the task store (msearch or other SO I/O).

## To verify
1. Apply the following diff, run Kibana and notice logs about poll
interval and capacity configuration changing
```
diff --git a/x-pack/plugins/task_manager/server/task_store.ts b/x-pack/plugins/task_manager/server/task_store.ts
index 2b3440e87c0..d2ffaa2f50f 100644
--- a/x-pack/plugins/task_manager/server/task_store.ts
+++ b/x-pack/plugins/task_manager/server/task_store.ts
@@ -574,6 +574,8 @@ export class TaskStore {
     const versionMap = this.createVersionMap([]);
     let allTasks = new Array<ConcreteTaskInstance>();

+    responses[0].status = 500;
+
     for (const response of responses) {
       if (response.status !== 200) {
         const err = new MsearchError(response.status);
```
2. Undo previous changes, apply the following diff, run Kibana and
notice logs about poll interval and capacity configuration changing
```
diff --git a/x-pack/plugins/task_manager/server/task_store.ts b/x-pack/plugins/task_manager/server/task_store.ts
index 2b3440e87c0..95d14152e1d 100644
--- a/x-pack/plugins/task_manager/server/task_store.ts
+++ b/x-pack/plugins/task_manager/server/task_store.ts
@@ -12,6 +12,7 @@ import murmurhash from 'murmurhash';
 import { v4 } from 'uuid';
 import { Subject } from 'rxjs';
 import { omit, defaults, get } from 'lodash';
+import { SavedObjectsErrorHelpers } from '@kbn/core/server';
 import { SavedObjectError } from '@kbn/core-saved-objects-common';

 import type * as estypes from '@elastic/elasticsearch/lib/api/typesWithBodyKey';
@@ -474,6 +475,7 @@ export class TaskStore {
   public async bulkGet(ids: string[]): Promise<BulkGetResult> {
     let result;
     try {
+      throw SavedObjectsErrorHelpers.decorateGeneralError(new Error('foo'));
       result = await this.savedObjectsRepository.bulkGet<SerializedConcreteTaskInstance>(
         ids.map((id) => ({ type: 'task', id }))
       );
```

x-pack/plugins/task_manager/server/lib/create_managed_configuration.test.ts

@@ -185,6 +185,17 @@ describe('createManagedConfiguration()', () => {
         expect(subscription).toHaveBeenNthCalledWith(2, 8);
       });
 
+      test('should decrease configuration at the next interval when a 500 error is emitted', async () => {
+        const { subscription, errors$ } = setupScenario(10);
+        errors$.next(SavedObjectsErrorHelpers.decorateGeneralError(new Error('a'), 'b'));
+        clock.tick(ADJUST_THROUGHPUT_INTERVAL - 1);
+        expect(subscription).toHaveBeenCalledTimes(1);
+        expect(subscription).toHaveBeenNthCalledWith(1, 10);
+        clock.tick(1);
+        expect(subscription).toHaveBeenCalledTimes(2);
+        expect(subscription).toHaveBeenNthCalledWith(2, 8);
+      });
+
       test('should decrease configuration at the next interval when a 503 error is emitted', async () => {
         const { subscription, errors$ } = setupScenario(10);
         errors$.next(SavedObjectsErrorHelpers.createGenericNotFoundEsUnavailableError('a', 'b'));
@@ -247,6 +258,17 @@ describe('createManagedConfiguration()', () => {
         expect(subscription).toHaveBeenNthCalledWith(2, 8);
       });
 
+      test('should decrease configuration at the next interval when an msearch 500 error is emitted', async () => {
+        const { subscription, errors$ } = setupScenario(10);
+        errors$.next(new MsearchError(500));
+        clock.tick(ADJUST_THROUGHPUT_INTERVAL - 1);
+        expect(subscription).toHaveBeenCalledTimes(1);
+        expect(subscription).toHaveBeenNthCalledWith(1, 10);
+        clock.tick(1);
+        expect(subscription).toHaveBeenCalledTimes(2);
+        expect(subscription).toHaveBeenNthCalledWith(2, 8);
+      });
+
       test('should decrease configuration at the next interval when an msearch 503 error is emitted', async () => {
         const { subscription, errors$ } = setupScenario(10);
         errors$.next(new MsearchError(503));
@@ -338,6 +360,16 @@ describe('createManagedConfiguration()', () => {
       expect(subscription).toHaveBeenNthCalledWith(2, 120);
     });
 
+    test('should increase configuration at the next interval when a 500 error is emitted', async () => {
+      const { subscription, errors$ } = setupScenario(100);
+      errors$.next(SavedObjectsErrorHelpers.decorateGeneralError(new Error('a'), 'b'));
+      clock.tick(ADJUST_THROUGHPUT_INTERVAL - 1);
+      expect(subscription).toHaveBeenCalledTimes(1);
+      clock.tick(1);
+      expect(subscription).toHaveBeenCalledTimes(2);
+      expect(subscription).toHaveBeenNthCalledWith(2, 120);
+    });
+
     test('should increase configuration at the next interval when a 503 error is emitted', async () => {
       const { subscription, errors$ } = setupScenario(100);
       errors$.next(SavedObjectsErrorHelpers.createGenericNotFoundEsUnavailableError('a', 'b'));

x-pack/plugins/task_manager/server/lib/create_managed_configuration.ts

@@ -165,8 +165,10 @@ function countErrors(errors$: Observable<Error>, countInterval: number): Observa
         (e) =>
           SavedObjectsErrorHelpers.isTooManyRequestsError(e) ||
           SavedObjectsErrorHelpers.isEsUnavailableError(e) ||
+          SavedObjectsErrorHelpers.isGeneralError(e) ||
           isEsCannotExecuteScriptError(e) ||
           getMsearchStatusCode(e) === 429 ||
+          getMsearchStatusCode(e) === 500 ||
           getMsearchStatusCode(e) === 503
       )
     )