…l-classes

Adding Bigtable factories and shell classes.

* feat: add support for soft delete

* add restore, get object, list_objects, unit tests

* integration test

* update restore_blob

* SoftDeletePolicy data class

* update docstrings; address comments

…emetry traces (#1229)

* all: implement custom tracer_provider injection

An important feature for observability is to allow the injection
of a custom tracer_provider instead of always using the global
tracer_provider by sending in

    observability_options=dict(
        tracer_provider=tracer_provider,
        enable_extended_tracing=True,
    )

* Address review feedback by attaching observability_options to Client only

* Attach observability_options directly before trace_call

* More reverts for formatting

* Plumb observability_options into _restart_on_unavailable

* completely decouple observability_options from session

* apply SPANNER_ENABLE_EXTENDED_TRACING but in inverse due to compatibility

* Document SPANNER_ENABLE_EXTENDED_TRACING in environment

* Revert a vestige of mock

* tests: add unit test for propagating TracerProvider

* Add preliminary end-to-end test to check for injection of observability_options

* Document default enable_extended_tracing value

* Carve out observability_options test

* Ensure that observability_options test sets up and deletes database

* Ensure instance.create() is invoked in system tests

* Use getattr for mock _Client

* Update with code review suggestions

* Deal with mock.Mock false positives failing tests

* Address review feedback