Add MetricOptions to simplify all addXGauge

[mayurkale22]

The list of options are too long for Gauges (Cumulative metrics - in the future) metrics and most of them are optional. It makes sense to use MetricOptions type to maintain the options. Also, in the future will be adding resource information to the MetricOptions list. This way it is easier to shrink or expand the input options.

Although, this is a breaking change in existing Gauge API but we don't expect too many users using this (released in last version - 0.0.9).

Specs: https://github.com/census-instrumentation/opencensus-specs/pull/248/files

[codecov-io]

Codecov Report

Merging #458 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   94.63%   94.69%   +0.06%     
==========================================
  Files         150      150              
  Lines        9628     9739     +111     
  Branches      722      727       +5     
==========================================
+ Hits         9111     9222     +111     
  Misses        517      517

Impacted Files	Coverage Δ
test/test-instana.ts	53.84% <0%> (-34.62%)	⬇️
src/common-utils.ts	95.83% <0%> (-4.17%)	⬇️
src/tracecontext-format.ts	96.07% <0%> (-0.36%)	⬇️
test/test-tracecontext-format.ts	98.91% <0%> (-0.16%)	⬇️
src/grpc-stats/server-stats.ts	100% <0%> (ø)	⬆️
src/grpc-stats/client-stats.ts	100% <0%> (ø)	⬆️
src/metrics/metric-registry.ts	100% <0%> (ø)	⬆️
test/test-prometheus-stats.ts	100% <0%> (ø)	⬆️
src/grpc-stats/stats-common.ts	100% <0%> (ø)	⬆️
test/test-stackdriver-monitoring.ts	95.31% <0%> (ø)	⬆️
... and 3 more

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