Update 'HTTP Client Metrics' reference documentation

Since gh-#5594, the 'outcome' tag has been introduced for
HTTP clients. This commit updates the reference documentation
accordingly.

Closes gh-15725

spring-boot-project/spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -1963,11 +1963,28 @@ customized by setting the `management.metrics.web.client.requests-metric-name` p
 By default, metrics generated by an instrumented client are tagged with the
 following information:
 
-* `method`, the request's method (for example, `GET` or `POST`).
-* `uri`,  the request's URI template prior to variable substitution, if possible (for
-example, `/api/person/{id}`).
-* `status`, the response's HTTP status code (for example, `200` or `500`).
-* `clientName`, the host portion of the URI.
+|===
+|Tag |Description
+
+|`clientName`
+|Host portion of the URI
+
+|`method`
+|Request's method (for example, `GET` or `POST`)
+
+|`outcome`
+|Request's outcome based on the status code of the response. 1xx is
+`INFORMATIONAL`, 2xx is `SUCCESS`, 3xx is `REDIRECTION`, 4xx `CLIENT_ERROR`, and 5xx is
+`SERVER_ERROR`
+
+|`status`
+|Response's HTTP status code (for example, `200` or `500`)
+
+|`uri`
+|Request's URI template prior to variable substitution, if possible (for example,
+`/api/person/{id}`)
+
+|===
 
 To customize the tags, and depending on your choice of client, you can provide
 a `@Bean` that implements `RestTemplateExchangeTagsProvider` or