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
public void testShouldBeGreenWhenAllPrimariesAndReplicasAreStarted() {
        var clusterState = createClusterStateWith(
            List.of(
                index("replicated-index", new ShardAllocation(randomNodeId(), AVAILABLE), new ShardAllocation(randomNodeId(), AVAILABLE)),
                index("unreplicated-index", new ShardAllocation(randomNodeId(), AVAILABLE))
            ),
            List.of()
        );
        var service = createAllocationHealthIndicatorService(clusterState);
        assertThat(
            service.calculate(true),
            equalTo(
                createExpectedResult(
                    GREEN,
                    "This cluster has all shards available.",
                    Map.of("started_primaries", 2, "started_replicas", 1),
                    Collections.emptyList()
                )
            )
        );
    }
    public void testShouldBeYellowWhenThereAreUnassignedReplicas() {
        var availableReplicas = randomList(0, 5, () -> new ShardAllocation(randomNodeId(), AVAILABLE));
        var unavailableReplicas = randomList(1, 5, () -> new ShardAllocation(randomNodeId(), UNAVAILABLE));
        var clusterState = createClusterStateWith(