Simplify SDK Configuration: Use View with default aggregation if no matching View is configured

[lalitb]

Fixes #1352

Changes

As of now, initial SDK configuration to create view for each instrument type can be complex as below:

 auto p = std::static_pointer_cast<metrics_sdk::MeterProvider>(provider);
  p->AddMetricReader(std::move(reader));

  // counter view
  std::string counter_name = name + "_counter";
  std::unique_ptr<metrics_sdk::InstrumentSelector> instrument_selector{
      new metrics_sdk::InstrumentSelector(metrics_sdk::InstrumentType::kCounter, counter_name)};
  std::unique_ptr<metrics_sdk::MeterSelector> meter_selector{
      new metrics_sdk::MeterSelector(name, version, schema)};
  std::unique_ptr<metrics_sdk::View> sum_view{
      new metrics_sdk::View{name, "description", metrics_sdk::AggregationType::kSum}};
  p->AddView(std::move(instrument_selector), std::move(meter_selector), std::move(sum_view));

  // histogram view
  std::string histogram_name = name + "_histogram";
  std::unique_ptr<metrics_sdk::InstrumentSelector> histogram_instrument_selector{
      new metrics_sdk::InstrumentSelector(metrics_sdk::InstrumentType::kHistogram, histogram_name)};
  std::unique_ptr<metrics_sdk::MeterSelector> histogram_meter_selector{
      new metrics_sdk::MeterSelector(name, version, schema)};
  std::unique_ptr<metrics_sdk::View> histogram_view{
      new metrics_sdk::View{name, "description", metrics_sdk::AggregationType::kHistogram}};
  p->AddView(std::move(histogram_instrument_selector), std::move(histogram_meter_selector),
             std::move(histogram_view));
  metrics_api::Provider::SetMeterProvider(provider);

Most of the time, the SDK devs just need to use the default aggregation for each of the instruments created by APIs. The PR simplifies the SDK configuration so that there is no need to add views for the instruments if the desired aggregation is the default one.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1358 (a89fdc2) into main (3a4a3a3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1358      +/-   ##
==========================================
+ Coverage   91.75%   91.76%   +0.02%     
==========================================
  Files         219      219              
  Lines        8092     8093       +1     
==========================================
+ Hits         7424     7426       +2     
+ Misses        668      667       -1     

Impacted Files	Coverage Δ
sdk/include/opentelemetry/sdk/metrics/view/view.h	77.78% <ø> (+11.12%)	⬆️
...ude/opentelemetry/sdk/metrics/view/view_registry.h	93.75% <100.00%> (ø)
sdk/test/metrics/view_registry_test.cc	100.00% <100.00%> (ø)

[esigo]

LGTM
Thanks for the PR