[proto] Add Prometheus Protobuf definitions to support annotations

[jeromefroe]

What this PR does / why we need it:

This PR adds Protobuf definitions for annotated datapoints to the m3prometheus package. These types will be part of an experimental API for supporting exemplars. I chose to create new types instead of augmenting the existing Sample type because support for annotations will be experimental to start and may undergo significant revisions. Introducing a new type may result in some code duplication but will help us ensure that the changes we make for annotations can be independent from the established types for Prometheus remote read and write.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[codecov]

Codecov Report

Merging #2029 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2029   +/-   ##
======================================
  Coverage    72.3%   72.3%           
======================================
  Files        1003    1003           
  Lines       86612   86612           
======================================
  Hits        62659   62659           
  Misses      19730   19730           
  Partials     4223    4223

Flag	Coverage Δ
#aggregator	82.1% <0%> (ø)	⬆️
#cluster	84.3% <0%> (ø)	⬆️
#collector	63.7% <0%> (ø)	⬆️
#dbnode	51.6% <0%> (ø)	⬆️
#m3em	53.3% <0%> (ø)	⬆️
#m3ninx	69.4% <0%> (ø)	⬆️
#m3nsch	49% <0%> (ø)	⬆️
#metrics	29.2% <0%> (ø)	⬆️
#msg	74.9% <0%> (ø)	⬆️
#query	68.9% <0%> (ø)	⬆️
#x	83.4% <0%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #2029 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2029   +/-   ##
======================================
  Coverage    72.3%   72.3%           
======================================
  Files        1004    1004           
  Lines       86613   86613           
======================================
  Hits        62671   62671           
  Misses      19720   19720           
  Partials     4222    4222

Flag	Coverage Δ
#aggregator	74.1% <0%> (ø)	⬆️
#cluster	83.7% <0%> (ø)	⬆️
#collector	63.5% <0%> (ø)	⬆️
#dbnode	51.6% <0%> (ø)	⬆️
#m3em	53.3% <0%> (ø)	⬆️
#m3ninx	69.4% <0%> (ø)	⬆️
#m3nsch	49% <0%> (ø)	⬆️
#metrics	29.2% <0%> (ø)	⬆️
#msg	74.9% <0%> (ø)	⬆️
#query	68.9% <0%> (ø)	⬆️
#x	83.4% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1461ce6...b1e08c1. Read the comment docs.

[codecov]

Codecov Report

Merging #2029 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2029   +/-   ##
======================================
  Coverage    72.8%   72.8%           
======================================
  Files        1006    1006           
  Lines       86164   86164           
======================================
  Hits        62742   62742           
  Misses      19211   19211           
  Partials     4211    4211

Flag	Coverage Δ
#aggregator	72% <0%> (ø)	⬆️
#cluster	40.2% <0%> (ø)	⬆️
#collector	68.9% <0%> (ø)	⬆️
#dbnode	75.5% <0%> (ø)	⬆️
#m3em	72.7% <0%> (ø)	⬆️
#m3ninx	15% <0%> (ø)	⬆️
#m3nsch	46.6% <0%> (ø)	⬆️
#metrics	56.7% <0%> (ø)	⬆️
#msg	74.9% <0%> (ø)	⬆️
#query	60.7% <0%> (ø)	⬆️
#x	67.2% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9906249...65adc1b. Read the comment docs.

[robskillington]

That sounds like a sane approach.

[robskillington]

LGTM