[Performance] Avoid duplication of metric exporters.

[marcalff]

When exporting metrics at different frequencies, the only possible setup according to the current spec is to:

• define multiple periodic metric readers, with different frequencies
• define a metric exporter dedicated for each reader

When all the exporters points to the same endpoint, this creates overhead:

• in memory, with multiple exporters send buffers
• in CPU, with multiple SSL sessions between the application and the endpoint
• in network, with multiple separate TCP/IP connections to the endpoint

To fix this, it should be possible to setup the SDK so that multiple metric readers can share a common metric exporter.

The spec however mentions that each reader owns one exporter, preventing sharing.

This performance improvement is blocked by the following spec change:

• https://github.com/open-telemetry/opentelemetry-specification/issues/3616