Add float histograms and gauge histograms to proto spec

Note that this is only an extension of the proto spec. Both generators
and consumers of the protobuf still need changes to make use of these
changes.

Gauge histograms measure current distributions. For one, they are
inspired by the GaugeHistogram type introducted by OpenMetrics, see
https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#gaugehistogram

They are also handled in the same way as OpenMetrics does it, by
using a new MetricType enum field GAUGE_HISTOGRAM, but not changing
anything else, i.e. for both regular and gauge histograms, the same
Histogram message type is used.

The other reason why we need gauge histograms comes from PromQL: If
you `rate` a histogram (which is possible with the new sparse
histograms as 1st class data type), the result is a gauge histogram. A
rate'd histogram can be created by a recording rule and then stored in
the TSDB. From there, it can be exposed by federation, so we need to
be able to represent it in the exposition format.

Float histograms are histograms where all counts (count of
observations, counts in each bucket, zero bucket count) are floating
point numbers rather than integer numbers. They are rarely needed for
direct instrumentation. Use cases are weighted histograms or timing
histograms, see kubernetes/kubernetes#109277
for a real-world example.

However, float histograms happen all the time as results of PromQL
expressions. Following the same line of argument as above, those float
histograms can end up in the TSDB via recording rules, which means
they can be exposed via federation.

Note that float histograms are implicitly supported by the original
Prometheus text format, as this format simply uses floating point
numbers for all sample values. OpenMetrics has avoided this ambiguity
and has specified integers for bucket counts and the count of
observations in a histogram, which means it needs to be extended to
support float histograms, similar to how this commit extends the
original Prometheus protobuf format.

Signed-off-by: beorn7 <[email protected]>