feat(docs): Add sequence diagram metrics flow

Update README.md to include the sequence diagram with a brief explanation of the flow

Signed-off-by: ​/Thor(sten)?/ Schwesig <[email protected]>

architecture/observability/README.md

@@ -39,6 +39,21 @@ The architecture diagram below is a graphical overview of the observability solu
 
 ![plot](./img/Observability-Architecture-v2.png)
 
+```mermaid
+sequenceDiagram
+    Prometheus->>Thanos: All Metrics
+    Thanos->>ODF: All Metrics
+    Thanos->>VictoriaMetrics: All Metrics
+    External Grafana-->>Thanos: Query A
+    Thanos-->>Thanos: Query A
+    Thanos-->>Thanos: A Metrics Age<6h
+    Thanos-->>ODF: Query A
+    ODF-->>Thanos: A 6h<Metrics Age<90d
+    Thanos-->>VictoriaMetrics: Query A
+    VictoriaMetrics-->>Thanos: A 90d<Metrics Age<1y
+    Thanos-->>External Grafana: Answer A
+```
+
 # Technology
 
 The following technologies have been chosen for this solution: