Restore tenant configuration for Loki forwarder.

This configuration will not be required for our default Loki instance,
but may be required for custom Loki instances.

enhancements/cluster-logging/forward-to-loki.md

@@ -148,6 +148,12 @@ The following optional output fields are Loki-specific:
   Example: `kubernetes.labels.foo` => `kubernetes_labels_foo`.\
   **Note**: `kubernetes.host` is *always* be included, even if not requested.
   It is required to ensure ordered label streams.
+- `tenantKey`: (string, default=`"kubernetes.namespaceName"`) \
+  Use the value of this meta-data key as the Loki tenant ID.\
+  At least these keys are supported:
+  - `kubernetes.namespaceName`: Use the namespace name as the tenant ID.
+  - `kubernetes.labels.<key>`: Use the string value of kubernetes label with key `<key>`.
+  - `openshift.labels.<key>`: use the value of a label attached by the forwarder.
 
 The full set of meta-data keys is listed in [data model][].
 
@@ -160,11 +166,24 @@ Notes on plug-in configuration:
 - Security: configured by the `output.secret` as usual.
 - K8s labels as Loki labels: supported by the plug-in.
 - Always include `kubernetes.host` to avoid avoid out-of-order streams.
+- Tenant set from `output.tenantKey`
 - Output format is `json`, serializes the fluentd record like other outputs.
 - Static labels set as `extra_labels` to avoid extracting from each record.
 
 ### User Stories
 
+#### Treat each namespace as a separate Loki tenant
+
+I want logs from each namespace to be directed to separate Loki tenants.
+
+``` yaml
+- name: myLokiOutput
+  type: loki
+  url: ...
+  secret: ...
+  tenantKey: kubernetes.namespace_name
+```
+
 #### Query all logs from a namespace
 
 ``` logql
@@ -243,16 +262,11 @@ For example a CI test cluster that constantly creates and destroys randomly-name
 
 ### Upgrade / Downgrade Strategy
 
-Need to provide migration assistance for users coming from Elasticsearch.
-Out of scope for this proposal.
-
-### Version Skew Strategy
+None.
 
 ## Implementation History
-## Drawbacks
-
-## Alternatives
 
+None.
 
 [label names]: https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels
 [data model]: https://github.com/openshift/origin-aggregated-logging/blob/master/docs/com.redhat.viaq-openshift-project.asciidoc