docs: Add URL config behavior spec

docs/specs/tsd-007-url-config-behavior.md

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+# URL-Based Config Behavior
+
+## Objective
+
+Define the retry and reload behavior of remote URLs that are passed as config to
+Telegraf. In terms of retry, currently Telegraf will attempt to load a remote
+URL three times and then exit. In terms of reload, Telegraf does not have the
+capability to reload remote URL based configs. This spec seeks to allow for
+options for the user to further these capabilities.
+
+## Keywords
+
+config, error, retry, reload
+
+## Overview
+
+Telegraf allows for loading configurations from local files, directories, and
+files via a URL. In order to allow situations where a configuration file is not
+yet available or due to a flaky network, the first proposal is to introduce a
+new CLI flag: `--url-config-retry-attempts`. This flag would continue to default
+to three and would specify the number of retries to attempt to get a remote URL
+during the initial startup of Telegraf.
+
+```sh
+--config-url-retry-attempts=3 Number of times to attempt to obtain a remote
+ configuration via a URL during startup. Set to
+ -1 for unlimited attempts.
+```
+
+These attempts would block Telegraf from starting up completely until success or
+until we have run out of attempts and exit.
+
+Once Telegraf is up and running, users can use the `--watch` flag to enable
+watching local files for changes and if/when changes are made, then reload
+Telegraf with the new configuration. For remote URLs, I propose a new CLI flag:
+`--url-config-check-interval`. This flag would set an internal timer that when
+it goes off, would check for an update to a remote URL file.
+
+```sh
+--config-url-watch-interval=0s Time duration to check for updates to URL based
+ configuration files. Disabled by default.
+```
+
+At each interval, Telegraf would send an HTTP HEAD request to the configuration
+URL, here is an example curl HEAD request and output:
+
+```sh
+$ curl --head http://localhost:8000/config.toml
+HTTP/1.0 200 OK
+Server: SimpleHTTP/0.6 Python/3.12.3
+Date: Mon, 29 Apr 2024 18:18:56 GMT
+Content-type: application/octet-stream
+Content-Length: 1336
+Last-Modified: Mon, 29 Apr 2024 11:44:19 GMT
+```
+
+The proposal then is to store the last-modified value when we first obtain the
+file and compare the value at each interval. No need to parse the value, just
+store the raw string. If there is a difference, trigger a reload.
+
+If anything other than 2xx response code is returned from the HEAD request,
+Telegraf would print a warning message and retry at the next interval. Telegraf
+will continue to run the existing configuration with no change.
+
+If the value of last-modified is empty, while very unlikely, then Telegraf would
+ignore this configuration file. Telegraf will print a warning message once about
+the missing field.
+
+## Relevant Issues
+
+* Configuration capabilities to retry for loading config via URL #[8854][]
+* Telegraf reloads URL-based/remote config on a specified interval #[8730][]
+
+[8854]: https://github.com/influxdata/telegraf/issues/8854
+[8730]: https://github.com/influxdata/telegraf/issues/8730