fix(inputs.mysql): Use tls=tlsid instead of tls=custom

Before mysql.ParseDSN(), replace "tls=custom" by "tls={tlsid}" in the DSN string so it matches the TLS Config name that was created earlier.
In order to replace it, first find the last "/" (slash) symbol and split the string in two slices, the second one being the "query part". Then, on the query part, find and replace "tls=custom" by "tls={tlsid}".
It is important to first split the string by the last "/" so if a username or password are "tls=custom", they are not replaced.

plugins/inputs/mysql/mysql.go

@@ -115,6 +115,18 @@ func (m *Mysql) Init() error {
 		}
 		dsn := dsnSecret.String()
 		dsnSecret.Destroy()
+		// We need to replace "tls=custom" in the DSN to use the tlsid generated instead.
+		// If not replaced, mysql.ParseDSN would look for a TLS Config named "custom" and
+		// would crash as the TLS Config needs to be present before parsing the DSN.
+		lastSlashIndex := strings.LastIndex(dsn, "/")
+		if lastSlashIndex != -1 {
+			dsnBase := dsn[:lastSlashIndex]   
+			dsnQuery := dsn[lastSlashIndex:]  
+			// Replace "tls=custom" with "tls={tlsid}" in the query part
+			dsnQuery = strings.Replace(dsnQuery, "tls=custom", "tls="+tlsid, 1)
+			// Join the parts back together
+			dsn = dsnBase + dsnQuery
+		}
 		conf, err := mysql.ParseDSN(dsn)
 		if err != nil {
 			return fmt.Errorf("parsing %q failed: %w", dsn, err)