doc(adr): add ADR regarding Option reconciliation (#10262)

Author: scotthart

doc/adr/2022-11-16-option-precedence-hierarchy.md

@@ -0,0 +1,32 @@
+**Title**: Establish how multiple values for the same option are reconciled.
+
+**Status**: proposed
+
+**Context**: There are a variety of ways that options can be customized for a
+given function call. Both `Client` and `Connection` classes can be created with
+`google::cloud::Options`. Individual function calls in a `Client` can be passed
+`google::cloud::Options`. Some environment variables can be set such that the
+variable value sets an option associated with that environment variable. Lastly,
+the storage library predates the `google::cloud::Options` mechanism, and many of
+the functions in this library consume a parameter pack that sets optional values
+for the RPCs used in the function.
+
+When the same option is set, with different values, via more than one of these
+mechanisms, there needs to be a uniform process of determining which value to
+use.
+
+**Decision**: The following priorities will be followed when determining which
+value to use when the same option is set via multiple mechanisms:
+
+1. Environment variables
+1. Per function call/GCS `Request` instance (if both are set with different values, an error is returned)
+1. `<service>Client` constructor `Options`
+1. `Make<service>Connection` factory function `Options`
+1. Default values
+
+**Consequences**: The intent was always to follow the priority defined here.
+Thus:
+
+- Failure to detect conflicts is a bug in the library.
+- Any changes in behavior are bug fixes.
+- The number of conflicts is expected to be small as the overlap is tiny and the features are rarely used.