[release-2.9.x] Docs: LogQL `offset` modifier (#10973)

Backport 86455195f3 from #10960

---

Bring back offset modifier doc section from
https://github.com/grafana/loki/pull/3455 and lost in
https://github.com/grafana/loki/pull/4012

**What this PR does / why we need it**:

**Which issue(s) this PR fixes**:
Fixes regression from https://github.com/grafana/loki/issues/2785

**Special notes for your reviewer**:

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [x] Documentation added
- [ ] Tests updated **N/A**
- [ ] `CHANGELOG.md` updated  **N/A**
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label **N/A**
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md` **N/A**
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e3ec)
**N/A**

Co-authored-by: Kevin Burek <kb000@users.noreply.github.com>

docs/sources/query/metric_queries.md

@@ -57,6 +57,15 @@ Examples:
     sum by (host) (rate({job="mysql"} |= "error" != "timeout" | json | duration > 10s [1m]))
     ```
 
+#### Offset modifier
+The offset modifier allows changing the time offset for individual range vectors in a query.
+
+For example, the following expression counts all the logs within the last ten minutes to five minutes rather than last five minutes for the MySQL job. Note that the `offset` modifier always needs to follow the range vector selector immediately.
+```logql
+count_over_time({job="mysql"}[5m] offset 5m) // GOOD
+count_over_time({job="mysql"}[5m]) offset 5m // INVALID
+```
+
 ### Unwrapped range aggregations
 
 Unwrapped ranges uses extracted labels as sample values instead of log lines. However to select which label will be used within the aggregation, the log query must end with an unwrap expression and optionally a label filter expression to discard [errors]({{< relref ".#pipeline-errors" >}}).