[chore] Enable generate-contrib step in check-contrib (open-telem…

…etry#12371)

#### Description

This PR enables the `make generate-contrib` step in `make check-contrib`
originally introduced in open-telemetry#11670, which allows testing the latest version
of the mdatagen against contrib. It had been disabled as initial
remediation against open-telemetry#11795, which has since been resolved by making
`make gotidy` on contrib tidy modules in topological order.

#### Link to tracking issue
Fixes open-telemetry#11167

#### Testing
Modifying `mdatagen` locally to crash on startup causes `make
check-contrib` to fail.

Makefile

@@ -270,6 +270,8 @@ check-contrib:
 		$(addprefix -replace ,$(join $(ALL_MOD_PATHS:%=go.opentelemetry.io/collector%=),$(ALL_MOD_PATHS:%=$(CURDIR)%)))"
 	@$(MAKE) -j2 -C $(CONTRIB_PATH) gotidy
 
+	@$(MAKE) generate-contrib
+
 	@echo -e "\nRunning tests"
 	@$(MAKE) -C $(CONTRIB_PATH) gotest