docs: update docs for loki.process with regex fields

Author: timonegk

CHANGELOG.md

@@ -22,6 +22,8 @@ Main (unreleased)
 
 - Add htpasswd file based authentication for `otelcol.auth.basic` (@pkarakal)
 
+- Add a `regex` argument to the `logfmt` and `json` stages in `loki.process` to extract log fields matching a regular expression. (@timonegk)
+
 ### Enhancements
 
 - update promtail converter to use `file_match` block for `loki.source.file` instead of going through `local.file_match`. (@kalleep)

docs/sources/reference/components/loki/loki.process.md

@@ -496,13 +496,16 @@ The following arguments are supported:
 
 | Name             | Type          | Description                                           | Default | Required |
 | ---------------- | ------------- | ----------------------------------------------------- | ------- | -------- |
-| `expressions`    | `map(string)` | Key-value pairs of JMESPath expressions.              |         | yes      |
+| `expressions`    | `map(string)` | Key-value pairs of JMESPath expressions.              |         | no       |
+| `regex`          | `string`      | Regular expression matched against JSON keys.         |         | no       |
 | `drop_malformed` | `bool`        | Drop lines whose input can't be parsed as valid JSON. | `false` | no       |
 | `source`         | `string`      | Source of the data to parse as JSON.                  | `""`    | no       |
 
 The `expressions` field is the set of key-value pairs of JMESPath expressions to run.
 The map key defines the name with which the data is extracted, while the map value is the expression used to populate the value.
 
+The `regex` field is a regular expression. All keys in the JSON source matching the regular expression are extracted.
+
 When configuring a JSON stage, the `source` field defines the source of data to parse as JSON.
 By default, this is the log line itself, but it can also be a previously extracted value.
 
@@ -653,19 +656,19 @@ The `stage.logfmt` inner block configures a processing stage that reads incoming
 
 The following arguments are supported:
 
-| Name      | Type          | Description                                    | Default | Required |
-| --------- | ------------- | ---------------------------------------------- | ------- | -------- |
-| `mapping` | `map(string)` | Key-value pairs of `logmft` fields to extract. |         | yes      |
-| `source`  | `string`      | Source of the data to parse as `logfmt`.       | `""`    | no       |
+| Name      | Type          | Description                                     | Default | Required |
+| --------- | ------------- | ----------------------------------------------- | ------- | -------- |
+| `mapping` | `map(string)` | Key-value pairs of `logmft` fields to extract.  |         | no       |
+| `regex`   | `string`      | Regular expression matched against logfmt keys. |         | no       |
+| `source`  | `string`      | Source of the data to parse as `logfmt`.        | `""`    | no       |
 
-The `source` field defines the source of data to parse as `logfmt`.
-When `source` is missing or empty, the stage parses the log line itself, but it can also be used to parse a previously extracted value.
+The `mapping` field is the set of key-value pairs.
+The map key defines the name with which the data is extracted, while the map value is the logfmt field used to populate the value.
 
-This stage uses the [go-logfmt][] unmarshaler, so that numeric or boolean types are unmarshalled into their correct form.
-The stage doesn't perform any other type conversions.
-If the extracted value is a complex type, it's treated as a string.
+The `regex` field is a regular expression. All logfmt fields matching the regular expression are extracted.
 
-[go-logfmt]: https://github.com/go-logfmt/logfmt
+The `source` field defines the source of data to parse as `logfmt`.
+When `source` is missing or empty, the stage parses the log line itself, but it can also be used to parse a previously extracted value.
 
 The following log line and stages demonstrates how this works.