You could run eigrp or ospf between the two locations. The bandwidth difference would be a highly decisive factor meaning that it's going to choose the 100Mb link for its primary routes (there are other deciding factors, but this is one of them). On your 10Mb interface, you should change the bandwidth to 10000 and the other 100Mb should be set to 100000. Let eigrp and ospf converge and tweak from there.
To use sla, you're going to need to have a data license for the universal images.
For traffic from the DC to site A i agree with John, use a routing protocol and have site A advertise it's subnets to the DC. The 100Mbps link should then be preferred if you are using EIGRP/OSPF.
However for traffic from site A to the DC it's not quite so clear cut. It depends on what sits behind the 2900 routers. If it is a L2 switch then you really have no choice but to use HSRP. But there is no way of tieing HSRP into OSPF ie. you simply make the 100Mbps lin 2900 HSRP active and if it fails then you switch to the other router. Here you may need tracking.
If however you have a L3 switch behind the 2900 a much better solution is to use routed point to point links between the 2900s and the L3 switch and send the routes received from the DC to that switch.
Edit - i'm not that familiar with Metro Ethernet but i'm assuming if the 3750 interface connecting to the 100Mbps link goes down then the 2900 at site A connecting to the 100Mbps link still thinks the link is okay. If the 2900 can see the link has gone down then no need for tracking obviously.
If you do have a L2 switch then you could form an EIGRP/OSPF neighborship between the 2900s on the internal interfaces. If you did this then each would see the others routes but obviously the 100Mbps link routes are preferred.
So the 100Mbps 2900 is HSRP active. If the Metro link fails the router no longer receives routes direct from the 3750 but it has routes via the 10Mbps router. So it stays active but simply sends packets from internal clients via the L2 switch to the other router and they are routed to the DC.
If the LAN interface fails then the neigborship is broken the HSRP active switches over. This works as well because now the 10Mbps router only has it's own routes.
So you could just use a routing protocol / HSRP and not worry about tracking.
If you had a L3 switch i would still exchange routes via L3 routed links as described before.
So basically John was totally correct in the first place (+5) and i have just added a lot of extra stuff which wasn't needed