querying multiple metrics using sum by

[amit kumar]

i have 3 metrics

metrics_1 , metrics_2 , metrics_3  all are of type histogram i want to write a query that gives the sum of all these metrics filtered by some label label_1

i have written this expression

(sum by (label_1)(metrics_1{label_1=~".+"} or vector(0)))+(sum by (label_1)(metrics_2{label_1=~".+"} or vector(0))) + (sum by (label_1)(metrics_3{label_1=~".+"} or vector(0)))

the issue i am facing is suppose metrics_1 is null and other have some valid data  then the whole expression is giving output as zero , what i am expecting is it should add the other 2 metrics and take metrics_1  as 0 and give the overall sum.

eg : 

metrics_1 = null

metrics_2 = 10

metrics_3 = 14

result = 24 (expected) but actual == 0

i read some blog that suggested to use coalesce ,fillna but both of these function is supported in my case

any help how can i deal with this ??

or is there any other way i can achieve my goal?