[receiver/expvar] Enable re-aggregation feature (open-telemetry#46979)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Enabling re-aggregation for the expvar receiver.
PR solves:
open-telemetry#46354
Part of:
open-telemetry#45396.

<!--Describe what testing was performed and which tests were added.-->
#### Testing
- Ran `make mdatagen` to regenerate the internal metadata code.
- Updated `config_test.go` & `scraper_test.go`
- Validated `make test` and `make lint`

Author: Anadee11

.chloggen/46354-enable-reaggregation.yaml

@@ -0,0 +1,11 @@
+change_type: enhancement
+
+component: receiver/expvar
+
+note: Enable the re-aggregation feature for the expvar receiver
+
+issues: [45396]
+
+subtext:
+
+change_logs: [user]

receiver/expvarreceiver/config_test.go

@@ -86,7 +86,34 @@ func TestLoadConfig(t *testing.T) {
 			}
 			assert.NoError(t, xconfmap.Validate(cfg))
 			if diff := cmp.Diff(tt.expected, cfg,
-				cmpopts.IgnoreUnexported(metadata.MetricConfig{}),
+				cmpopts.IgnoreUnexported(
+					metadata.ProcessRuntimeMemstatsBuckHashSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsFreesMetricConfig{},
+					metadata.ProcessRuntimeMemstatsGcCPUFractionMetricConfig{},
+					metadata.ProcessRuntimeMemstatsGcSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsHeapAllocMetricConfig{},
+					metadata.ProcessRuntimeMemstatsHeapIdleMetricConfig{},
+					metadata.ProcessRuntimeMemstatsHeapInuseMetricConfig{},
+					metadata.ProcessRuntimeMemstatsHeapObjectsMetricConfig{},
+					metadata.ProcessRuntimeMemstatsHeapReleasedMetricConfig{},
+					metadata.ProcessRuntimeMemstatsHeapSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsLastPauseMetricConfig{},
+					metadata.ProcessRuntimeMemstatsLookupsMetricConfig{},
+					metadata.ProcessRuntimeMemstatsMallocsMetricConfig{},
+					metadata.ProcessRuntimeMemstatsMcacheInuseMetricConfig{},
+					metadata.ProcessRuntimeMemstatsMcacheSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsMspanInuseMetricConfig{},
+					metadata.ProcessRuntimeMemstatsMspanSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsNextGcMetricConfig{},
+					metadata.ProcessRuntimeMemstatsNumForcedGcMetricConfig{},
+					metadata.ProcessRuntimeMemstatsNumGcMetricConfig{},
+					metadata.ProcessRuntimeMemstatsOtherSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsPauseTotalMetricConfig{},
+					metadata.ProcessRuntimeMemstatsStackInuseMetricConfig{},
+					metadata.ProcessRuntimeMemstatsStackSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsSysMetricConfig{},
+					metadata.ProcessRuntimeMemstatsTotalAllocMetricConfig{},
+				),
 				cmpopts.IgnoreUnexported(configoptional.Optional[configauth.Config]{}),
 				cmpopts.IgnoreUnexported(configoptional.Optional[confighttp.CookiesConfig]{}),
 			); diff != "" {