Hi
Heart rate readings can be taken manually via the Heart rate app:
Check your heart rate
It is not possible to manually configure the frequency with which Apple Watch takes background heart rate measurements.
Readings are not taken at fixed five minute intervals. They are instead taken automatically at intervals that will vary, depending on your activity level, as explained in the following support article. Readings could, for example, be taken automatically as frequently as every one, two or three minutes:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Heart rate is automatically recorded continuously during workouts when recording them via the built-in Workout app and third-party fitness tracking apps that support the feature:
Work out with your Apple Watch - Apple Support
Recording heart rate continuously at other times would result in battery drain. Examples of battery life under different uses are provided here:
Apple Watch - Battery - Apple
If you would like to submit a feature request to Apple, then you can do so here:
Feedback - Apple Watch - Apple