[connector/spanmetrics] fix(docs): add example of R.E.D metrics for s…

…pan data (#29115)

**Description:** This is to clarify the metrics name generated.

**Link to tracking Issue:**
[26520](#26520)

**Testing:** n/a docs

**Documentation:** n/a

connector/spanmetricsconnector/README.md

@@ -31,11 +31,23 @@ Aggregates Request, Error and Duration (R.E.D) OpenTelemetry metrics from span d
 dimensions, including Errors. Multiple metrics can be aggregated if, for instance,
 a user wishes to view call counts just on `service.name` and `span.name`.
 
+```
+calls{service.name="shipping",span.name="get_shipping/{shippingId}",span.kind="SERVER",status.code="Ok"}
+```
+
 **Error** counts are computed from the Request counts which have an `Error` Status Code metric dimension.
 
+```
+calls{service.name="shipping",span.name="get_shipping/{shippingId},span.kind="SERVER",status.code="Error"}
+```
+
 **Duration** is computed from the difference between the span start and end times and inserted into the
 relevant duration histogram time bucket for each unique set dimensions.
 
+```
+duration{service.name="shipping",span.name="get_shipping/{shippingId}",span.kind="SERVER",status.code="Ok"}
+```
+
 Each metric will have _at least_ the following dimensions because they are common
 across all spans: