Metadata.Add example:
// Add metadata to the dictionary by calling the Add method
metadata.Add("docType", "textDocuments");
SetMetadataAsync example:
// Set the blob's metadata.
await blob.SetMetadataAsync(metadata);
// Set the blob's properties.
await blob.SetPropertiesAsync();
Reference:
https://docs.microsoft.com/en-us/azure/storage/blobs/storage-blob-properties-metadata