[Metrics API] Update asynchronous instruments to not allow calling observe outside of a callback

[utpilla]

All the asynchronous instruments implement the trait AsyncInstrument which allows users to call observe method outside of the callback

let observable_counter = meter
        .u64_observable_counter("my_observable_counter")
        .with_callback(|observer| {
            observer.observe(
                100,
                &[
                    KeyValue::new("mykey1", "myvalue1"),
                    KeyValue::new("mykey2", "myvalue2"),
                ],
            )
        })
        .init();

observable_counter.observe(500, &[]); // This should not be allowed

Changes

• This PR removes the implementation of AsyncInstrument for each of the asynchronous instruments to not allow calling observe outside of a callback (as a side benefit, we also reduce the public API surface)
• That also means that we can completely remove AsyncInstrument from each of asynchronous instruments as it was only used for implementation of itself

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes

[codecov]

Codecov Report

Attention: Patch coverage is 64.28571% with 10 lines in your changes missing coverage. Please review.

Project coverage is 79.2%. Comparing base (16c0e10) to head (aaea102).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry/src/metrics/mod.rs	0.0%	7 Missing ⚠️
opentelemetry-sdk/src/metrics/meter.rs	66.6%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2210     +/-   ##
=======================================
+ Coverage   79.1%   79.2%   +0.1%     
=======================================
  Files        122     122             
  Lines      21042   21009     -33     
=======================================
- Hits       16645   16644      -1     
+ Misses      4397    4365     -32     

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[cijothomas]

Lets mention in changelog as this is breaking (though unlikely to affect anyone in practice)

[ThomsonTan]

nit: Declare the full type name of PhantomData in use?