Custom labels/constants in Alertmanager config - variables

Question

I have an Alertmanager config file which defines many alerts. All of these alerts use the same time range in their Prometheus expressions and messaging. Is there way I can define a constant once in the file, which is then used throughout. That way, if I ever change the value of this constant, it's only required to be changed in one place?
Here's an illustration of what I am trying to achieve:
apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
name: kube-prometheus-stack-services
namespace: monitoring
labels:
app: kube-prometheus-stack
release: kube-prometheus-stack
customer_variables:
time_range: 5m # CUSTOM VARIABLE DEFINED HERE
spec:
groups:
- name: Service1
rules:
- alert: Service1Alert1
annotations:
summary: "Some summary of alert1"
message: "Some error rate has increased to {{ $value }}% in the last {{ $customer_variables.time_range }}." # CUSTOM VARIABLE USED HERE
expr: # some expression using my_metric_1{label="..."}[{{ $customer_variables.time_range }}] # CUSTOM VARIABLE USED HERE
for: {{ $customer_variables.time_range }} # CUSTOM VARIABLE USED HERE
labels:
severity: critical
- alert: Service1Alert2
annotations:
summary: "Some summary of alert2"
message: "Some other error rate has increased to {{ $value }}% in the last {{ $customer_variables.time_range }}." # CUSTOM VARIABLE USED HERE
expr: # some expression using my_metric_1{label="..."}[{{ $customer_variables.time_range }}] # CUSTOM VARIABLE USED HERE
for: {{ $customer_variables.time_range }} # CUSTOM VARIABLE USED HERE
labels:
severity: critical