one get ennobled in the Klingon society? In (most of) ours, this happens when a man of means provides military resources for a man of highest political power, in such an amount that he himself doesn't yet become a man of highest political power. But Klingons are all warriors (at least through several prominent members of each clan or bloodline or whatnot), and Martok did have a House, even if not a noble one, that brought its resources to the Empire's struggles.
Is Klingon nobility a closed community of limited numbers, thanks to it really never needing any refreshment from among people who can factually provide military resources? In our society, nobility soon becomes incapable of performing its original duty, this duty ironically being too common for them, yet in the Klingon society this would not appear to be the case.
Martok was a commoner officer, but even "Once More Unto" did not make it sound as if this would be, ahem, uncommon. Indeed, for all we know, most Klingon officers might be commoners, and for this reason inferior to those who are not, formal rank aside. A Klingon commands by his own choosing and through his personal strength, not by mandate from above. Apparently, a noble Klingon just adds
the mandate from above to this to enjoy superior status when really big decisions are made.
Martok's father apparently had never been an officer, nor had any of his ancestors. But Martok becoming one through bravery did not turn him into a nobleman. It is left unclear whether Martok would have been ennobled had he reached his commission through attending the Academy, which apparently produced "elite officer ranks". Such a thing would IMHO fit the Klingon mindset, though: a nobleman would create himself though martial achievements, but only if the right people approved.