Instrument queries for CloudSQL Query Insights

[lutter]

For the hosted service, it would probably help if indexing was instrumented with tags for query insights We should think through the kinds of analyses this enables, and instrument queries accordingly.

I am thinking of adding the following as tags to queries:

• the subgraph ID
• the block number
• the operation/action the query belongs to (e.g., insert/update/delete/get/get_many)
• the entity type

We'll have to map those to the tags that CloudSQL supports, but that's just going to be a convention.

This instrumentation should be possible to turn on/off with an environment variable

[lutter]

Part of this (for GraphQL queries) is done in #4079