Remove gauge from the API

[bogdandrutu]

Left a TODO to update the comment for Meter (comment is very old).

[jkwatson]

looks like a checkstyle failure on the build

[codecov-io]

Codecov Report

Merging #830 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #830      +/-   ##
============================================
+ Coverage     78.96%   79.05%   +0.09%     
+ Complexity      798      790       -8     
============================================
  Files           106      105       -1     
  Lines          2885     2845      -40     
  Branches        275      273       -2     
============================================
- Hits           2278     2249      -29     
+ Misses          503      493      -10     
+ Partials        104      103       -1

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/opentelemetry/sdk/metrics/MeterSdk.java	86.66% <ø> (+10.19%)	10 <0> (ø)	⬇️
...in/java/io/opentelemetry/metrics/DefaultMeter.java	71.18% <ø> (-1.49%)	25 <0> (-6)
...elemetry/opentracingshim/SpanContextShimTable.java	96.42% <0%> (+7.14%)	8% <0%> (+1%)	⬆️

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