Fix broken Nrpe dashboard (#389)

* conditional popping

* bump

lib/charms/grafana_k8s/v0/grafana_dashboard.py

@@ -219,7 +219,7 @@ def __init__(self, *args):
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
 
-LIBPATCH = 40
+LIBPATCH = 41
 
 PYDEPS = ["cosl >= 0.0.50"]
 
@@ -633,7 +633,10 @@ def _replace_template_fields(  # noqa: C901
         deletions = []
         for tmpl in dict_content["templating"]["list"]:
             if tmpl["name"] and tmpl["name"] in used_replacements:
-                deletions.append(tmpl)
+                # it might happen that existing template var name is the same as the one we insert (i.e prometheusds or lokids)
+                # in that case, we want to pop the existing one only.
+                if tmpl not in DATASOURCE_TEMPLATE_DROPDOWNS:
+                    deletions.append(tmpl)
 
         for d in deletions:
             dict_content["templating"]["list"].remove(d)