[receiver/nginx] Enable attribute reaggregation (#47489)

This PR enables the re-aggregation feature for the `nginx` receiver by:

1. Setting `reaggregation_enabled: true` in `metadata.yaml`
2. Marking the `state` attribute as `recommended` so it can be disabled
by users for reaggregation

The `state` attribute (active/reading/writing/waiting) is used only by
`nginx.connections_current`. Marking it `recommended` means it is
enabled by default (preserving current behavior) but can be disabled via
config to aggregate across all states.

Generated files under `internal/metadata/` were regenerated with `go
generate ./...` followed by `goimports -w` (as `make generate` does at
the root).

Fixes #46368
Part of #45396

---------

Signed-off-by: Ogulcan Aydogan <ogulcanaydogan@hotmail.com>

Author: ogulcanaydogan

.chloggen/46368-nginx-enable-reaggregation.yaml

@@ -0,0 +1,6 @@
+change_type: enhancement
+component: receiver/nginx
+note: Enable the re-aggregation feature for the nginx receiver
+issues: [46368]
+subtext:
+change_logs: [user]

receiver/nginxreceiver/internal/metadata/config.schema.yaml

@@ -18,6 +18,22 @@ $defs:
           enabled:
             type: boolean
             default: true
+          aggregation_strategy:
+            type: string
+            enum:
+              - "sum"
+              - "avg"
+              - "min"
+              - "max"
+            default: "sum"
+          attributes:
+            type: array
+            items:
+              type: string
+              enum:
+                - "state"
+            default:
+              - "state"
       nginx.connections_handled:
         description: "NginxConnectionsHandledMetricConfig provides config for the nginx.connections_handled metric."
         type: object