Clarify that prometheus bridge does not support views

I noticed some confusion on this point in https://docs.google.com/document/d/1ZaFr8qbCpOZqnX7ItnLLg6S4NMwF3SiUuM5yBm4sIYY/edit#heading=h.v38n4whtu270.

We should clarify in the limitations section of the README that the bridge does not support views.