eg
```
"impacts": [
            {
              "severity": 3,
              "description": "Searches might return slower than usual. Fewer redundant copies of the data exist on 2 indices [.ds-logslogslogs-2022.03.29-000001, my-index-000001].",
              "impact_areas": [ "search" ]
            }
          ]
```
server/src/test/java/org/elasticsearch/cluster/routing/allocation/ShardsAvailabilityHealthIndicatorServiceTests.java
Map.of("red-index", 3, "yellow-index-1", 5, "yellow-index-2", 8)
        );
        var clusterState = createClusterStateWith(
            List.of(
                index("red-index", new ShardAllocation(randomNodeId(), UNAVAILABLE), new ShardAllocation(randomNodeId(), AVAILABLE)),
                index("yellow-index-1", new ShardAllocation(randomNodeId(), AVAILABLE), new ShardAllocation(randomNodeId(), UNAVAILABLE)),
                index("yellow-index-2", new ShardAllocation(randomNodeId(), AVAILABLE), new ShardAllocation(randomNodeId(), UNAVAILABLE))
            ),
            List.of(),
            indexMetadataList
        );
        var service = createAllocationHealthIndicatorService(clusterState);
        HealthIndicatorResult result = service.calculate(true);
        assertEquals(RED, result.status());
        assertEquals("This cluster has 1 unavailable primary, 2 unavailable replicas.", result.summary());
        assertEquals(2, result.impacts().size());
        assertEquals(
            result.impacts().get(0),
            new HealthIndicatorImpact(1, "Cannot add data to 1 index [red-index]. Searches might return incomplete results.")
        );
        
        assertThat(
            result.impacts().get(1),
            equalTo(
                new HealthIndicatorImpact(
                    3,
                    "Searches might return slower than usual. Fewer redundant copies of the data exist on 2 indices [yellow-index-2, "
                        + "yellow-index-1]."
                )
            )