[Schema Registry Avro] Log cache size (Azure#20733)

### Packages impacted by this PR
@azure/schema-registry-avro

### Issues associated with this PR
Fixes Azure#20245
Fixes Azure#20363
Fixes Azure#20565

### Describe the problem that is addressed by this PR
Logging cache size so customers understands how it grows in their application

### What are the possible designs available to address the problem? If there are more than one possible design, why was the one in this PR chosen?
The proposed approach logs the length of all stringified schemas which I believe is consistent with what .NET does.

What else could we log here that we should consider?


### Are there test cases added in this PR? _(If not, why?)_
N/A

### Provide a list of related PRs _(if any)_
N/A

### Command used to generate this PR:**_(Applicable only to SDK release request PRs)_

### Checklists
- [x] Added impacted package name to the issue description
- [ ] Does this PR needs any fixes in the SDK Generator?** _(If so, create an Issue in the [Autorest/typescript](https://github.com/Azure/autorest.typescript) repository and link it here)_
- [ ] Added a changelog (if necessary)

Author: deyaaeldeen