docs(env): first cut at describing SDK environment variables

env.md

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+# Agoric SDK Environment Variables
+
+```js
+process.env.DOCUMENTATION // for power users
+```
+
+This file describes environment variables that influence the execution of the
+Agoric SDK.
+
+<!--
+## Template
+
+**Add to a new section in alphabetical order.**
+
+Affects:
+
+Purpose:
+
+Description:
+
+Lifetime:
+
+-->
+
+## ALLOW_IMPLICIT_REMOTABLES
+
+Affects: marshal
+
+Purpose: to control whether the marshal system demands `Far`
+
+Description: set to `true` if you need to debug problems with missing `Far`
+declarations.
+
+Lifetime: until all sources (including dapps) conform to using `Far`
+declarations for all remote objects
+
+## DEBUG
+
+Affects: agoric (CLI), ag-chain-cosmos, ag-solo
+
+Purpose: to change the meaning of `console.log` and other console methods
+
+Description: uses `anylogger` to change whether the following methods (in order
+of increasing severity) are active for a given context:
+
+1. `console.debug`
+2. `console.log`
+3. `console.info`
+4. `console.warn`
+5. `console.error`
+
+If not set, then default (`console.info` and above) logging is enabled.
+(`console.log` and `console.debug` logging is disabled.)
+
+If set to an empty string, or running in `ag-chain-cosmos start` mode, don't
+print any logs.  This is part of "consensus mode."
+
+If set to a value that contains the substring `agoric`, then print all console
+messages for the entire SDK.
+
+Otherwise, set to a comma-separated list of prefixes, where each prefix is the
+context given to `makeConsole`.  For example:
+
+- `DEBUG=SwingSet:ls` enable all console messages for liveslots, regardless of vat.
+- `DEBUG=SwingSet:ls:v13` enable for liveslots in vat 13.
+- `DEBUG=SwingSet:vat` enable for user code, regardless of vat.
+- `DEBUG=SwingSet:vat,SwingSet:ls` enable for liveslots and user code, all vats
+
+## END_BLOCK_SPIN_MS
+
+Affects: cosmic-swingset
+
+Purpose: simulating load to explore scalability of the Cosmos SDK.
+
+Description: Number of milliseconds to busy-wait during every block's
+end-of-block processing for `ag-chain-cosmos` and the sim-chain.
+
+Lifetime: until we have a more consistent load testing regimen
+
+## NO_FAKE_CURRENCIES
+
+Affects: cosmic-swingset
+
+Purpose: skipping the creation of pretend tokens
+
+Description: When nonempty, don't create pretend prepopulated tokens like
+"moola" or "simoleans".
+
+Lifetime: until chain is mature enough not to need any pretend tokens
+
+## OTEL_EXPORTER_PROMETHEUS_HOST
+## OTEL_EXPORTER_PROMETHEUS_PORT
+
+Affects: cosmic-swingset
+
+Purpose: enabling Prometheus metrics exports
+
+Description: When either is set, these are the host IP and port number to use
+for the Prometheus scrape endpoint to export telemetry.
+
+Lifetime: until we decide not to support Prometheus for metrics export
+
+## SOLO_SLOGFILE
+
+Same as SLOGFILE, but for solo instead of chain.
+
+Lifetime: ?
+
+## SLOGFILE
+
+Affects: cosmic-swingset
+
+Purpose: record an on-chain Swingset LOG file
+
+Description: when nonempty, use the value as an absolute path to which SwingSet
+debug logs should be written.
+
+Lifetime: ?