Make necessary metrics API symbols private

Fixes open-telemetry#2622

Author: ocelotl

docs/api/metrics.instrument.rst

@@ -1,7 +1,7 @@
 opentelemetry._metrics.instrument
 =================================
 
-.. automodule:: opentelemetry._metrics.instrument
+.. automodule:: opentelemetry._metrics._instrument
     :members:
     :private-members:
     :undoc-members:

docs/api/metrics.observation.rst

@@ -1,7 +1,7 @@
 opentelemetry._metrics.observation
 ==================================
 
-.. automodule:: opentelemetry._metrics.observation
+.. automodule:: opentelemetry._metrics._observation
     :members:
     :undoc-members:
     :show-inheritance:

docs/api/metrics.rst

@@ -3,7 +3,7 @@ opentelemetry._metrics package
 
 .. warning::
    OpenTelemetry Python metrics are in an experimental state. The APIs within
-   :mod:`opentelemetry._metrics` are subject to change in minor/patch releases and make no
+   :mod:`opentelemetry._metrics.___init__` are subject to change in minor/patch releases and make no
    backward compatability guarantees at this time.
 
    Once metrics become stable, this package will be be renamed to ``opentelemetry.metrics``.
@@ -19,4 +19,4 @@ Submodules
 Module contents
 ---------------
 
-.. automodule:: opentelemetry._metrics
+.. automodule:: opentelemetry._metrics.___init__