Support OTLP metrics v0.10 data point flags (i.e., staleness)

[jmacd]

Is your feature request related to a problem? Please describe.

OTLP Metrics v0.10 supports a new flags field in four of its data point types and a new optional flag FLAG_NO_RECORDED_VALUE used to indicate a point that is not present. This is intended for Prometheus receiver and exporter to communicate about staleness in a specified way. Prior to v0.10, staleness would be indicated by a NaN value in the sum or gauge, which was problematic for histogram streams where only the Sum field contains a suitable floating point field. Prior to v0.10, an integer-valued NumberDataPoint would have to switch to floating point values to express NaN values as well.

Describe the solution you'd like
Add model/pdata support for DataPointFlags, FLAG_NO_RECORDED_VALUE, and the associated four flags.

Describe alternatives you've considered
n/a

Additional context
This PR open-telemetry/opentelemetry-proto#316 added the feature to the protocol.

Issues about staleness in the prometheus-wg repository have been closed using the workarounds described above. Now OTLP senders and receivers have a specified way to achieve the same functionality.