net-mgmt/telegraf: Add header config to OpenTelemetry output and fix service_address string

[tlaerm]

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• [] I opened an issue first for non-trivial changes and linked it below.
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Describe the problem

My OpenTelemetry collector is running on a https endpoint and requires a bearer token to be passed with each ingestion request. With 1.12.14 it is impossible to set this header in the config even though telegraf itself provides for it. see https://github.com/influxdata/telegraf/tree/master/plugins/outputs/opentelemetry

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Describe the proposed solution

This PR adds two fields to the OpenTelemetry output config: One for the header name and one for the header value. I considered other formats, maybe even a possibility to add multiple headers but this all seemed overkill and would make the UI and input processing way more complex. This should solve the problem for the vast majority of users.
The other change is an even more trivial fix, adding quotes on the service_address field value as otherwise it does not work with https urls like https://hostname.tld/path/to/collector . The help text now also reflects the correct usage.

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Related issue

None, consider it a trivial change.