Increment a streamz counter with the specified metric and field
names.
Metric names should start with a '/', generally be lowercase-only,
and end in "_count". Field names should not contain an initial
slash. The actual exported metric names will have "/iam/policy"
prepended.
Field names correspond to IAM request parameters and field values
are their respective values.
Supported field names: - "authority", which is "[token]" if
IAMContext.token is present, otherwise the value of
IAMContext.authority_selector if present, and otherwise a
representation of IAMContext.principal; or - "iam_principal", a
representation of IAMContext.principal even if a token or authority
selector is present; or - "" (empty string), resulting in a counter
with no fields.