Add memcached to db.system semantic conventions (open-telemetry#1689)

* Add memcached to db.system semantic conventions

As we already use `memcached` to represent the `db.system` Memcached in opentelemetry-python, I believe we should add to the specification recommendation https://github.com/open-telemetry/opentelemetry-python-contrib/blob/e606542981580971af1bbecbf07671c39f799665/instrumentation/opentelemetry-instrumentation-pymemcache/src/opentelemetry/instrumentation/pymemcache/__init__.py#L160

Author: ericmustin

CHANGELOG.md

@@ -23,6 +23,8 @@ release.
 
 - Add JSON RPC specific conventions ([#1643](https://github.com/open-telemetry/opentelemetry-specification/pull/1643)).
 
+- Add Memcached to Database specific conventions ([#1689](https://github.com/open-telemetry/opentelemetry-specification/pull/1689)).
+
 ### Compatibility
 
 ### OpenTelemetry Protocol

semantic_conventions/trace/database.yaml

@@ -147,6 +147,9 @@ groups:
               - id: elasticsearch
                 value: 'elasticsearch'
                 brief: 'Elasticsearch'
+              - id: memcached
+                value: 'memcached'
+                brief: 'Memcached'
         - id: connection_string
           tag: connection-level
           type: string

specification/trace/semantic_conventions/database.md

@@ -106,6 +106,7 @@ Some database systems may allow a connection to switch to a different `db.user`,
 | `neo4j` | Neo4j |
 | `geode` | Apache Geode |
 | `elasticsearch` | Elasticsearch |
+| `memcached` | Memcached |
 <!-- endsemconv -->
 
 ### Notes and well-known identifiers for `db.system`