Add a basic Long and Double gauge implementation

[thisthat]

This addresses #684 and it is implemented following #755

[codecov-io]

Codecov Report

Merging #764 into master will decrease coverage by 0.39%.
The diff coverage is 59.37%.

@@             Coverage Diff             @@
##             master     #764     +/-   ##
===========================================
- Coverage     78.84%   78.45%   -0.4%     
- Complexity      761      767      +6     
===========================================
  Files            98      100      +2     
  Lines          2709     2771     +62     
  Branches        255      265     +10     
===========================================
+ Hits           2136     2174     +38     
- Misses          473      495     +22     
- Partials        100      102      +2

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/opentelemetry/sdk/metrics/MeterSdk.java	41.66% <0%> (ø)	5 <0> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/SdkDoubleGauge.java	61.29% <61.29%> (ø)	3 <3> (?)
...ava/io/opentelemetry/sdk/metrics/SdkLongGauge.java	61.29% <61.29%> (ø)	3 <3> (?)

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[jkwatson]

thanks for getting this done!

[bogdandrutu]

Thank you for this PR @thisthat, I will hold on merging this PR until we discuss open-telemetry/opentelemetry-specification#412. If you think we should make progress we can merge it and maybe remove later if the decision will be to remove the Gauge instrument.

[thisthat]

I would argue to hold this PR until a decision at the specification level is taken. If the Gauge API will change/remove, I will update/delete this PR accordingly.

[bogdandrutu]

Based on the open-telemetry/oteps#80, I will close this PR, feel free to reopen if needed.