def logpdf_link(self, link_f, y, Y_metadata=None):
return -link_f + y*np.log(link_f) - special.gammaln(y+<NUM_LIT:1>)<EOL>
Log Likelihood Function given link(f)

.. math::
    \\ln p(y_{i}|\lambda(f_{i})) = -\\lambda(f_{i}) + y_{i}\\log \\lambda(f_{i}) - \\log y_{i}!

:param link_f: latent variables (link(f))
:type link_f: Nx1 array
:param y: data
:type y: Nx1 array
:param Y_metadata: Y_metadata which is not used in poisson distribution
:returns: likelihood evaluated for this point
:rtype: float