otel, loki: support dynamic headers()

[alltilla]

No description provided.

[github-actions]

This Pull Request introduces config grammar changes

axoflow/71be47e0d85d46e497584cf39d346164a7e0c89b -> alltilla/grpc-dynamic-headers

--- a/destination
+++ b/destination

 axosyslog-otlp(
+    frac-digits(<nonnegative-integer>)
     headers(
-        <string> => <string>
+        <string> => <template-content>
+        <string> => <template-reference>
     )
+    local-time-zone(<string>)
+    on-error(<string>)
+    send-time-zone(<string>)
+    template-escape(<yesno>)
+    time-zone(<string>)
+    ts-format(<string>)
 )

 bigquery(
     headers(
-        <string> => <string>
+        <string> => <template-content>
+        <string> => <template-reference>
     )
 )

 loki(
     headers(
-        <string> => <string>
+        <string> => <template-content>
+        <string> => <template-reference>
     )
 )

 opentelemetry(
+    frac-digits(<nonnegative-integer>)
     headers(
-        <string> => <string>
+        <string> => <template-content>
+        <string> => <template-reference>
     )
+    local-time-zone(<string>)
+    on-error(<string>)
+    send-time-zone(<string>)
+    template-escape(<yesno>)
+    time-zone(<string>)
+    ts-format(<string>)
 )

 syslog-ng-otlp(
+    frac-digits(<nonnegative-integer>)
     headers(
-        <string> => <string>
+        <string> => <template-content>
+        <string> => <template-reference>
     )
+    local-time-zone(<string>)
+    on-error(<string>)
+    send-time-zone(<string>)
+    template-escape(<yesno>)
+    time-zone(<string>)
+    ts-format(<string>)
 )

[bazsi]

I did not test this explicitly, but went through the commits and they all looked fine to me.