Getting Polyline (Linestring) coordinates from line layer in QGIS

I have a road network (line layer). I am trying to get list of coordinates (Lat,Long) from road network in the form of geometry LINESTRING as shown below. However, I am only getting two coordinates as shown in second figure.

What I have tried:

lyr = iface.activeLayer()
fl = ['lat','lon'] #Add these fields before executing code

with edit(lyr):
    for f in lyr.getFeatures():
        p = f.geometry().centroid().asPoint()
        _=f.setAttribute(f.fields().indexFromName(fl[0]), p.y())
        _=f.setAttribute(f.fields().indexFromName(fl[1]), p.x())
        _=lyr.updateFeature(f)

lyr = iface.activeLayer()

with edit(lyr):
    for f in lyr.getFeatures():
        f["geometry"] = f.geometry().asWkt()
        lyr.updateFeature(f)