Salesforce : Apex Http callout

HttpRequest query = new HttpRequest();
query.setMethod('GET');
query.setEndPoint('callout:service_pwd/users'); // Le nom d'utilisateur et le mot de passe sont définis dans le Named Credential service_pwd.
Http http = new http();
HttpResponse response = http.send(query);
String responseBody = response.getBody();
if(response.getStatusCode() == 200) {
    // Do something with responseBody.
} else {
    // Handle error.
}
HttpRequest query = new HttpRequest();
query.setMethod('POST');
query.setEndPoint('callout:client_auth'); // Le nom d'utilisateur et le mot de passe sont définis dans le Named Credential service_pwd.
query.setHeader('Content-Type', 'application/x-www-form-urlencoded');
query.setBody('grant_type=client_credentials&scope=scopes&audience=audiences' +'&'+ 'client_id='+'{!$Credential.Username}' +'&'+ 'client_secret=' + '{!$Credential.Password}');
Http http = new http();
HttpResponse response = http.send(query);
String responseBody = response.getBody();
if(response.getStatusCode() == 200) {
    Map<String, Object> resultsBody = (Map<String, Object>) JSON.deserializeUntyped(responseBody);
    String accessToken = resultsBody.get('access_token');
    HttpRequest resourceQuery = new HttpRequest();
    resourceQuery.setMethod('GET');
    resourceQuery.setEndPoint('callout:client_resource/resources');
    resourceQuery.setHeader('Authorization', 'Bearer ' + accessToken);
    Http resourceHttp = new http();
    HttpResponse resourceResponse = resourceHttp.send(resourceQuery);
    if(resourceResponse.getStatusCode() == 200) {
        // Handle Resource response
    } else {
        // Handle Error.
    }
} else {
    // Handle error.
}
public class HttpExt {
    public static final long TOKEN_VALID_FOR = 3600; // one hour.
    private static final String TEST_ENDPOINT = 'https://token-endpoint.com';
    private String method;
    private String endPoint;
    private final HttpRequest request;

    public HttpExt() {
        this.request = new HttpRequest();
    }

    public static JWTExchange__mdt getJWTConfig(String name) {
        // Retrieve JWTExchange__mdt by DeveloperName =: name
    }

    public static OAuth2Parameter__mdt getClientCredentialsConfig(String name) {
        // Retrieve OAuth2Parameter__mdt by DeveloperName =: name
    }

    public static String getJWTToken(JWTExchange__mdt config) {
        return ''; // JWT Token
    }

    public static String getAccessToken(String tokenUrl, String requestBody) {
        // Send HTTP request
    }

    public static String exchangeToken(JWTExchange__mdt config) {
        String JWTtoken = getJWTToken(config);
        String tokenUrl = config.TokenUrl__c;
        String body = 'grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=' + JWTtoken;
        return getAccessToken(tokenUrl, body);
    }

    /**
     * client_id and client_secret must be defined in NamedCredetials
     */
    public static String clientCredentialsToken(OAuth2Parameter__mdt config) {
        String tokenUrl = config.TokenUrl__c;
        String body = 'grant_type=client_credentials' +'&'+ 'client_id='+'{!$Credential.Username}' +'&'+ 'client_secret=' + '{!$Credential.Password}';
        if (String.isNotBlank(config.Scope__c)) {
            body += '&scope=' + config.Scope__c;
        }
        if (String.isNotBlank(config.Audience__c)) {
            body += '&audience=' + config.Audience__c;
        }
        return getAccessToken(tokenUrl, body);
    }

    public HttpResponse send() {
		String finalEndPoint = this.endPoint;
        if (finalEndPoint.startsWith('jwt:')) { // Start JWT Exchange
            finalEndPoint = finalEndPoint.substring(4);
            String name = finalEndPoint.split('/')[0];
            JWTExchange__mdt config = getJWTConfig(name);
            String accessToken = exchangeToken(config);
            request.setHeader('Authorization', 'Bearer ' + accessToken);
            finalEndPoint = 'callout:' + finalEndPoint;
        } else  if (finalEndPoint.startsWith('client_credentials:')) {
            finalEndPoint = finalEndPoint.substring(19);
            String name = finalEndPoint.split('/')[0];
            OAuth2Parameter__mdt config = getClientCredentialsConfig(name);
            String accessToken = clientCredentialsToken(config);
            request.setHeader('Authorization', 'Bearer ' + accessToken);
            finalEndPoint = 'callout:' + finalEndPoint;
        }
        // Add more protocol
        request.setEndpoint(finalEndPoint);
        request.setMethod(method);
        Http http = new Http();
        return http.send(request);
    }

    public class HttpExtException extends Exception {}
}
HttpExt query = new HttpExt();
query.setMethod('GET');
query.setEndPoint('client_credentials:client_resource/resources'); // Les Custom Metadata OAuth2Parameter__mdt doit avoir un enregistrement dont le DeveloperName est client_resource le même nom que le NC.
                                                                   // Le CM client_resource.TokenUrl__c doit être callout:client_auth
HttpResponse response = query.send();
String responseBody = response.getBody();
if(response.getStatusCode() == 200) {
    // Do something with responseBody.
} else {
    // Handle error.
}
HttpExt query = new HttpExt();
query.setMethod('GET');
query.setEndPoint('jwt:jwt_resource/resources'); // Les Custom Metadata JWTExchange__mdt doit avoir un enregistrement dont le DeveloperName est jwt_resource le même nom que le NC.
                                                 // Le CM jwt_resource.TokenUrl__c doit être callout:jwt_auth
HttpResponse response = query.send();
String responseBody = response.getBody();
if(response.getStatusCode() == 200) {
    // Do something with responseBody.
} else {
    // Handle error.
}