Add guidance about dropping support for older library versions (#4802)

* Add guidance about dropping support for older library versions

* Spotless

Author: trask

VERSIONING.md

@@ -31,3 +31,47 @@ Not all of our artifacts are published as stable artifacts - any non-stable arti
 `-alpha` on its version. NONE of the guarantees described above apply to alpha artifacts. They may
 require code or environment changes on every release and are not meant for consumption for users
 where versioning stability is important.
+
+## Dropping support for older library versions
+
+### Library instrumentation
+
+Bumping the minimum supported library version for library instrumentation is generally acceptable
+if there's a good reason because:
+
+* Users of library instrumentation have to integrate the library instrumentation during build-time
+  of their application, and so have the option to bump the library version if they are using an
+  unsupported library version.
+* Users have the option of staying with the old version of library instrumentation, without being
+  pinned on an old version of the OpenTelemetry API and SDK.
+* Bumping the minimum library version changes the artifact name, so it is not technically a breaking
+  change.
+
+### Javaagent instrumentation
+
+The situation is much trickier for javaagent instrumentation:
+
+* A common use case of the javaagent is applying instrumentation at deployment-time (including
+  to third-party applications), where bumping the library version is frequently not an option.
+* While users have the option of staying with the old version of javaagent, that pins them on
+  an old version of the OpenTelemetry API and SDK, which is problematic for the OpenTelemetry
+  ecosystem.
+* While bumping the minimum library version changes the instrumentation module name, it does not
+  change the "aggregated" javaagent artifact name which most users depend on, so could be considered
+  a breaking change for some users (though this is not a breaking change that we currently make any
+  guarantees about).
+
+For these reasons, bumping the minimum supported library version for a javaagent instrumentation
+requires more scrutiny and must be considered on a case-by-case basis.
+
+When there is functionality in a new library version that requires changes to the javaagent
+instrumentation which are incompatible with the current javaagent base library version, some options
+that do not require bumping the minimum supported library version include:
+
+* Access the new functionality via reflection. This is a good technique only for very small changes.
+* Create a new javaagent instrumentation module to support the new library version. This requires
+  configuring non-overlapping versions in the muzzle check and applying `assertInverse` to confirm
+  that the two instrumentations are never be applied to the same library version (see
+  [class loader matchers](https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/docs/contributing/writing-instrumentation-module.md#restrict-the-criteria-for-applying-the-instrumentation-by-extending-the-classloadermatcher-method)
+  for how to restrict instrumentations to specific library versions). If there's too much copy-paste
+  between the two javaagent instrumentation modules, a `-common` module can be extracted.