Tag values not visible from Kapacitor Template

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Hi,

We are using Template and YAML file to create alerts. Alerts are working fine but messages is missing information related to tags.
Following is the template that I have created

```
// /usr/share/kapacitor/templates/database_template.tick

var measurement string
var where_filter = lambda: TRUE
var groups = [*]
var field string
var warn lambda
var crit lambda
var period_val duration
var every_val duration
var email_id string
var message_val = '{{.Level}}: {{.TaskName}} at {{ index .Fields "mean" }} - Host: {{ index .Tags "host" }}'
var details_val = 'Alert Level - {{.Level}}
Alert Name - {{.TaskName}}
Current Value - {{ index .Fields "mean" }}
Tags - {{.Group}}
Host - {{ index .Tags "host" }}'

stream
|from()
.measurement(measurement)
.where(where_filter)
.groupBy(groups)
|window()
.period(period_val)
.every(every_val)
|mean(field)
|alert()
.warn(warn)
.crit(crit)
.message('{{ index .Tags "host" }}')
.details(details_val)
.email(email_id)
```

Following is the YAML file

```
# /usr/share/kapacitor/tasks/asm_test_alert.tick
dbrps:
- { db: "telegraf", rp: "autogen"}
template-id: database_template
vars:
measurement:
type: string
value: AsmSpaceUtilization
groups:
type: list
value:
- type: string
value: db_name
- type: string
value: dg_name
period_val:
type: duration
value: 10s
every_val:
type: duration
value: 10s
field:
type: string
value : used_pct
warn:
type: lambda
value: "\"mean\" > 40.0"
crit:
type: lambda
value: "\"mean\" > 50.0"
email_id:
type: string
value: "advaitvdeo@yahoo.com"
```

When I get email alert, I see following in subject

OK: asm_test_alert at 32 - Host:

Its not printing host value when I am using following reference in message - {{ index .Tags "host" }}

Email body shows following

Alert Level - OK
Alert Name - asm_test_alert
Current Value - 32
Tags - db_name=orcl,dg_name=ORCL_DG
Host -

Here also host value is not getting printed.
Am I referring the host tag correctly? or we have a different way to refer host tag?
Same thing works when I directly write TICK script instead of using template.

Regards

Start with /usr/share/kapacitor/templates/database_template.tick and /usr/share/kapacitor/tasks/asm_test_alert.tick, then compare their alert rendering with the direct TICK script that works. Trace how template alerts populate .Tags and verify the rendered subject and body include the host tag; done means the template produces the expected host value.

Written by the indexing model from the issue text.