The name after the `oneof` doesn't show up in the explicit getter for
a particular field, e.g. for the sum of a `HistogramValue` as a
double, the getter is called `GetDoubleValue` in Go. Nothing in this
getter tells you that you are getting the sum. For one, this is
confusing. But you will also get a name collision if any of the other
fields becomes a `oneof` (e.g. the count for a float histogram). With
this commit, the getter will be called `GetDoubleSum` etc.

Signed-off-by: beorn7 <beorn@grafana.com>

There are rare use cases of "weighted" or "scaled" histogram, where
the total count and the count in buckets are actually floating point
values.

The need for this "float histogram" becomes more obvious with the
introduction of Native Histograms. With Histograms being first class
citizens also within Prometheus, a recording rule that outputs
Histograms will generally output Histograms where the counts are
floating point numbers again (because PromQL operates on floats only,
and because the result of a `rate` operation is interpolated and
therefore not integral anymore, even if the input is). If such a
result of a recording rule is federated, we need to represent "float
histograms" in the exposition format.

This commit introduces the "float histogram" capability to the
conventional Histogram, which is also needed to add Native Histograms
later.

Signed-off-by: beorn7 <beorn@grafana.com>

This is still experimental. It is not clear yet how to represent
native histograms in the text format, but with this commit,
implementers of OpenMetrics using the protobuf format can already
gather experience with native histograms.

Signed-off-by: beorn7 <beorn@grafana.com>

This commit mirrors
prometheus/client_model#80 .

Signed-off-by: beorn7 <beorn@grafana.com>

The proto file was missing the `go_package` option.

Furthermore, the generated Go file was put into the `bin` directory,
which is weird.

Signed-off-by: beorn7 <beorn@grafana.com>