Rename metric instruments to match feature-freeze API specification

[jmacd]

This renames ValueRecorder to Histogram, ValueObserver to GaugeObserver, SumObserver to CounterObserver, and UpDownSumObserver to UpDownCounterObserver.

The API was marked "feature-freeze" here: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md

This updates getting-started.md with an up-to-date example of getting started with the metrics API.

This is otherwise not a functional change.

Note: The OTel-Go metrics API is still considered experimental and subject to change in the future. We discussed making this change ASAP to avoid confusing new users who review the API specification and will not understand these legacy names.

[codecov]

Codecov Report

Merging #2202 (cea4420) into main (1f527a5) will not change coverage.
The diff coverage is 58.5%.

@@          Coverage Diff          @@
##            main   #2202   +/-   ##
=====================================
  Coverage   72.9%   72.9%           
=====================================
  Files        176     176           
  Lines      12162   12162           
=====================================
  Hits        8868    8868           
  Misses      3055    3055           
  Partials     239     239           

Impacted Files	Coverage Δ
...tlp/otlpmetric/internal/otlpmetrictest/otlptest.go	0.0% <0.0%> (ø)
metric/metric_sdkapi.go	100.0% <ø> (ø)
metric/sdkapi/instrumentkind_string.go	40.0% <ø> (ø)
metric/metric.go	56.4% <50.0%> (ø)
metric/metric_instrument.go	86.4% <86.6%> (ø)
bridge/opencensus/exporter.go	100.0% <100.0%> (ø)
metric/sdkapi/instrumentkind.go	100.0% <100.0%> (ø)
sdk/export/metric/metric.go	44.6% <100.0%> (ø)
sdk/metric/aggregator/aggregator.go	100.0% <100.0%> (ø)
sdk/metric/selector/simple/simple.go	100.0% <100.0%> (ø)

[MrAlias]

Looks good overall with two minor nitpicks about the CHANGELOG. Nothing blocking.

[evantorrie]

Looks good vis. a vis. syntactical changes.

My comments are nearly all regarding the "human-readable" and variable names in tests. For consistency, I would prefer to see all the names of test instruments changed in the same way we've changed the code names, e.g. we shouldn't see

valuerecorder := ...

or

NewInt64Histogram("valuerecorder.int", ...)

in the test code. but rather

histogram := ...

and

NewInt64Histogram("histogram.int", ...)

The same comment applies to the other name changes.

[codeboten]

Renames look good, just one question regarding additive vs adding, but otherwise I agree this change will greatly reduce confusion.

[jmacd]

I removed the change in getting-started.md because @MrAlias's #2203 removes the example that would be changed.

[jmacd]

Thank you ❤️ @evantorrie for the fixes. I went through and found a few more and have generally tidied up the PR.

[evantorrie]

LGTM