fix: cherry-pick #1511 to 0.8.x: expose metrics for validator (#1511) (#1512)

cherry-pick PR #1511 from `develop`
## 🔄 Changes Summary
- This PR exposes metrics for the validator component by adding a call
to `metrics.Register()` in the `AggsenderValidator.Start()` method,
ensuring that validator-related Prometheus metrics (error counts,
invalid signatures, validation time) are properly registered when the
validator service starts.

## 🐞 Issues
- Closes #1510

## 🔗 Related PRs
-  #1511

Author: joanestebanr

aggsender/aggsender_validator.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 
 	"github.com/agglayer/aggkit/agglayer"
+	"github.com/agglayer/aggkit/aggsender/metrics"
 	"github.com/agglayer/aggkit/aggsender/types"
 	"github.com/agglayer/aggkit/aggsender/validator"
 	v1 "github.com/agglayer/aggkit/aggsender/validator/proto/v1"
@@ -56,6 +57,7 @@ func NewAggsenderValidator(ctx context.Context,
 	}, nil
 }
 func (a *AggsenderValidator) Start(ctx context.Context) {
+	metrics.Register()
 	a.validatorService.Start(ctx)
 }