correct span name in AbstractRpcBasedConnectionRegistry#getActiveMaster

It looks like that the span name does not correspond to what is actually done.
 
public CompletableFuture<ServerName> getActiveMaster()

{ return tracedFuture( () -> this .<GetActiveMasterResponse> call( (c, s, d) -> s.getActiveMaster(c, GetActiveMasterRequest.getDefaultInstance(), d), GetActiveMasterResponse::hasServerName, "getActiveMaster()") .thenApply(resp -> ProtobufUtil.toServerName(resp.getServerName())), getClass().getSimpleName() + ".getClusterId"); }