Docs: Add some jsdoc style comments to prometheus utf8 support for better clarity (#105862)

* add some jsdoc style comments for better clarity

* Update utf8_support.ts

packages/grafana-prometheus/src/utf8_support.ts

@@ -1,3 +1,17 @@
+/**
+ * Ensures a string is compatible with Prometheus' UTF-8 handling rules.
+ *
+ * Prometheus has specific rules for handling UTF-8 strings in metric names and label values:
+ * - Legacy names (matching pattern [a-zA-Z_:][a-zA-Z0-9_:]*) are used as-is
+ * - Non-legacy names containing UTF-8 characters must be wrapped in double quotes
+ *
+ * @param value - The string to make UTF-8 compatible
+ * @returns The original string if it's empty or a valid legacy name, otherwise the string wrapped in double quotes
+ *
+ * @example
+ * utf8Support('metric_name') // returns 'metric_name'
+ * utf8Support('metric-📈') // returns '"metric-📈"'
+ */
 export const utf8Support = (value: string) => {
   if (value === '') {
     return value;
@@ -9,6 +23,22 @@ export const utf8Support = (value: string) => {
   return `"${value}"`;
 };
 
+/**
+ * Escapes a string to make it compatible with Prometheus UTF-8 support.
+ *
+ * This function converts non-legacy name characters to an escaped format:
+ * - Underscores are doubled as '__'
+ * - Valid legacy runes are preserved as-is
+ * - Invalid code points are replaced with '_FFFD_'
+ * - Other characters are converted to '_HEX_' format where HEX is the hexadecimal code point
+ *
+ * @param value - The string to escape
+ * @returns An escaped string prefixed with 'U__' that is compatible with Prometheus
+ *
+ * @example
+ * escapeForUtf8Support("my lovely_http.status:sum") // returns U__my_20_lovely__http_2e_status:sum
+ * escapeForUtf8Support("label with 😱") // returns U__label_20_with_20__1f631_
+ */
 export const escapeForUtf8Support = (value: string) => {
   const isLegacyLabel = isValidLegacyName(value);
   if (isLegacyLabel) {
@@ -42,6 +72,16 @@ export const escapeForUtf8Support = (value: string) => {
   return escaped;
 };
 
+/**
+ * Checks if a string is a valid legacy (the standard) Prometheus metric or label name.
+ *
+ * Valid legacy (the standard) names match the pattern [a-zA-Z_:][a-zA-Z0-9_:]* which means:
+ * - First character must be a letter, underscore, or colon
+ * - Remaining characters can only be letters, numbers, underscores, or colons
+ *
+ * @param name - The string to check
+ * @returns true if the string is a valid legacy (the standard) name, false otherwise
+ */
 export const isValidLegacyName = (name: string): boolean => {
   if (name.length === 0) {
     return false;
@@ -57,9 +97,17 @@ export const isValidLegacyName = (name: string): boolean => {
   return true;
 };
 
-// const labelNamePriorToUtf8Support = /^[a-zA-Z_:][a-zA-Z0-9_:]*$/;
-// instead of regex we use rune check (converted from prometheus code)
-// https://github.com/prometheus/common/blob/main/model/metric.go#L426-L428
+/**
+ * Checks if a character is valid for a legacy (the standard) Prometheus metric or label name.
+ *
+ * This is an implementation of the Prometheus model rune validation logic, which
+ * determines if a character is allowed in a legacy metric or label name.
+ * https://github.com/prometheus/common/blob/v0.64.0/model/metric.go#L430-L432
+ *
+ * @param char - The character to check
+ * @param index - The position of the character in the string
+ * @returns true if the character is valid at the given position, false otherwise
+ */
 const isValidLegacyRune = (char: string, index: number): boolean => {
   const codePoint = char.codePointAt(0);
   if (codePoint === undefined) {
@@ -75,11 +123,26 @@ const isValidLegacyRune = (char: string, index: number): boolean => {
   );
 };
 
+/**
+ * Validates if a Unicode code point is valid for UTF-8 encoding.
+ *
+ * @param codePoint - The Unicode code point to validate
+ * @returns true if the code point is valid (between 0 and 0x10FFFF), false otherwise
+ */
 const isValidCodePoint = (codePoint: number): boolean => {
-  // Validate the code point for UTF-8 compliance if needed.
   return codePoint >= 0 && codePoint <= 0x10ffff;
 };
 
+/**
+ * Wraps each key in a Prometheus filter string with UTF-8 support.
+ *
+ * This function processes a filter string (e.g. 'metric="value",name=~"pattern"')
+ * and applies UTF-8 support to each key while preserving the operators and values.
+ * It handles quoted values and comma separators correctly.
+ *
+ * @param filterStr - The filter string to process
+ * @returns A new filter string with UTF-8 support applied to the keys
+ */
 export const wrapUtf8Filters = (filterStr: string): string => {
   const resultArray: string[] = [];
   const operatorRegex = /(=~|!=|!~|=)/; // NOTE: the order of the operators is important here