[ResponseOps] Updating TM metrics to handle when capacity estimation returns NaN (#207116)

Resolves https://github.com/elastic/kibana/issues/204467

## Summary

`assumedRequiredThroughputPerMinutePerKibana` is `NaN` when the
`capacityStats.runtime.value.load.p90` is undefined. This PR adds a
check to catch when the load.p90 is undefined, throw an error, and
ignore calculating the capacity estimation.


### Checklist

- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### To verify
I was not able to reproduce this locally without changing the code, so
here is how I tested the code and I am definitely open to suggestions of
how to better test this.

1. Update the code to set `capacityStats.runtime.value.load.p90:
undefined`. I set it
[here](286c9e2ddb/x-pack/platform/plugins/shared/task_manager/server/monitoring/capacity_estimation.ts (L55)),
but there are other places upstream where you could set it to
`undefined`.
2. Start Kibana
3. Verify that you see the following log message:
```
 Task manager had an issue calculating capacity estimation. averageLoadPercentage: undefined
```

x-pack/platform/plugins/shared/task_manager/server/monitoring/capacity_estimation.test.ts

@@ -8,6 +8,7 @@
 import { CapacityEstimationParams, estimateCapacity } from './capacity_estimation';
 import { HealthStatus, RawMonitoringStats } from './monitoring_stats_stream';
 import { mockLogger } from '../test_utils';
+import { AveragedStat } from './task_run_calculators';
 
 describe('estimateCapacity', () => {
   const logger = mockLogger();
@@ -833,6 +834,24 @@ describe('estimateCapacity', () => {
       max_throughput_per_minute: 200,
     });
   });
+  test('throws an error when the runtime load is undefined', async () => {
+    expect(
+      () =>
+        estimateCapacity(
+          logger,
+          mockStats(
+            {},
+            {},
+            {
+              load: {} as AveragedStat,
+            }
+          ),
+          2
+        ).value.observed
+    ).toThrowErrorMatchingInlineSnapshot(
+      `"Task manager had an issue calculating capacity estimation. averageLoadPercentage: undefined"`
+    );
+  });
 });
 
 function mockStats(

x-pack/platform/plugins/shared/task_manager/server/monitoring/capacity_estimation.ts

@@ -65,6 +65,11 @@ export function estimateCapacity(
     capacity: { config: configuredCapacity },
   } = capacityStats.configuration.value;
 
+  if (!averageLoadPercentage) {
+    throw new Error(
+      `Task manager had an issue calculating capacity estimation. averageLoadPercentage: ${averageLoadPercentage}`
+    );
+  }
   /**
    * On average, how many polling cycles does it take to execute a task?
    * If this is higher than the polling cycle, then a whole cycle is wasted as
@@ -265,10 +270,15 @@ export function withCapacityEstimate(
   assumedKibanaInstances: number
 ): RawMonitoringStats['stats'] {
   if (isCapacityEstimationParams(monitoredStats)) {
-    return {
-      ...monitoredStats,
-      capacity_estimation: estimateCapacity(logger, monitoredStats, assumedKibanaInstances),
-    };
+    try {
+      return {
+        ...monitoredStats,
+        capacity_estimation: estimateCapacity(logger, monitoredStats, assumedKibanaInstances),
+      };
+    } catch (e) {
+      // Return monitoredStats with out capacity estimation
+      logger.error(e.message);
+    }
   }
   return monitoredStats;
 }