Add a metric for missing blobs when staging local execution.

[deuszx]

In ChainClient::stage_block_execution we try to exeute a block locally and might get a LocalNodeError::BlobsNotFound as a result of that call. If that happens, we try to receive_certificates_for_blobs and then retry after finding the missing blob.

It would be useful to know how often that happens - i.e. how often a client tries to execute a block w/o having certificates for all the blobs in it.