[m3coordinator] Support annotations in writer

[jeromefroe]

What this PR does / why we need it:

This PR adds support to DownsamplerAndWriter for writing annotations. This will allow m3coordinator to pass annotations along to M3DB which can be used to, for example, store exemplars alongside datapoints.

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 #2022 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2022   +/-   ##
======================================
  Coverage    72.4%   72.4%           
======================================
  Files        1007    1007           
  Lines       86645   86645           
======================================
  Hits        62753   62753           
  Misses      19657   19657           
  Partials     4235    4235

Flag	Coverage Δ
#aggregator	75.5% <0%> (ø)	⬆️
#cluster	82.8% <0%> (ø)	⬆️
#collector	63.4% <0%> (ø)	⬆️
#dbnode	69.8% <0%> (ø)	⬆️
#m3em	73.2% <0%> (ø)	⬆️
#m3ninx	27.1% <0%> (ø)	⬆️
#m3nsch	51.1% <0%> (ø)	⬆️
#metrics	17.7% <0%> (ø)	⬆️
#msg	74.2% <0%> (ø)	⬆️
#query	70.1% <0%> (ø)	⬆️
#x	80.5% <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 83ccc30...29af098. Read the comment docs.

[robskillington]

LGTM