Syntax for using headers inside the client body of otelcol.exporter.otlphttp

I have defined a otelexporter in my pipeline but end with http 406 error, the documentation suggests that the file format is not known and hence the error. Now I have to pass ‘content-type:application/json’ in the header what should be the syntax? I tried below and it failed.

the documentation says the headers should be defined in the client block : otelcol.exporter.otlphttp | Grafana Agent documentation

otelcol.exporter.otlphttp "elastic" {
  client {
    endpoint = "https://elasticsearch:9200"
    auth = otelcol.auth.basic.elastic.handler
      ca_file = "/path/to/certs.pem"