Add entity count in graph health

[llunaCreixent]

It was deleted in #11,
but the visual component

import GrainIcon from '@material-ui/icons/Grain';

  <Chip
    icon={<GrainIcon />}
    label={`${health.graph.entityCount} entities`}
  />

and the complete query to the graph

{
  indexingStatusForCurrentVersion(subgraphName: "${process.env.SUBGRAPH_NAME}") {
    synced
    health
    entityCount
    fatalError {
      message
      block {
        number
        hash
      }
      handler
    }
    chains {
      chainHeadBlock {
        number
      }
      latestBlock {
        number
      }
    }
  }
}

can be added back once the index-node requests work again on thegraph.com.

[llunaCreixent]

index-node requests are working again!