Historiography is the study of the historical process - historical methods, biases, authorship, etc. Metafiction is also sometimes referred to as illusory fiction, fiction that may not be fiction, fiction that has elements of reality, fiction that blurs the connection between what is real and what is not real. I know, it sounds convoluted.
Linda Hutcheon, a Canadian academic and literary critic, coined this term. It means, in her words, works that "are well-known and popular novels which are both intensely self-reflexive and yet paradoxically also lay claim to historical events and personages".
The novel is sometimes referred to as a "non-fiction novel" because it is based on events that actually occured (the murder of the Clutter family in Kansas by two ex-convicts, Dick Hickock and Perry Smith). Capote read about the murders in the newspaper and became fascinated with it. He went to Kansas to interview everyone involved, taking along his childhood friend, Harper Lee. It took him years to finally finish the novel and the end result was criticized by some because subsequent investigations and interviews by other literary critics and writers found conflicting information and accused Capote of making up some of the details, conversations, etc. They said he had grown too close to the convicts.
Even though Hickock and Smith were brutal murderers, Capote presented them in almost a likeable manner. He spent years interviewing them, following the case, etc., and he got to know them very well. He became friends with them and some critics have stated that Capote was therefore unable to separate their likeability from their perverse behavior. Further, both of the convicts had sexual issues and were always trying to prove themselves to each other. A homosexual himself, some critics argue that this was an underlying reason for the favorable presentation of the convicts by Capote. He could identify with them, hence the "reflexive" nature of the metafiction.
Because the lines between what happened historically and Capote's presentation of what happened were blurred, we can say that the way Capote characterized Hickock and Smith was an example of historiographic metafiction because what we know happened doesn't really jive with what we feel about the characters due to Capote's sympathetic depiction of them in the novel.
There is a good discussion of this novel on eNotes. See the link below.