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
List.of(
                index(
                    "yellow-index",
                    new ShardAllocation(randomNodeId(), AVAILABLE),
                    concatLists(availableReplicas, unavailableReplicas).toArray(ShardAllocation[]::new)
                )
            ),
            List.of()
        );
        var service = createAllocationHealthIndicatorService(clusterState);
        assertThat(
            service.calculate(true),
            equalTo(
                createExpectedResult(
                    YELLOW,
                    unavailableReplicas.size() > 1
                        ? "This cluster has " + unavailableReplicas.size() + " unavailable replicas."
                        : "This cluster has 1 unavailable replica.",
                    Map.of(
                        "started_primaries",
                        1,
                        "unassigned_replicas",
                        unavailableReplicas.size(),
                        "started_replicas",
                        availableReplicas.size()
                    ),
                    List.of(
                        new HealthIndicatorImpact(
                            3,
                            "Searches might return slower than usual. Fewer redundant copies of the data exist on 1 index [yellow-index]."